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A Mechanistic Interpretability Analysis of Grokking

18Neel Nanda

17David Schneider-Joseph

9Daniel Kokotajlo

6Neel Nanda

4Neel Nanda

5Ben Lang

4Neel Nanda

7gwern

10Gunnar Zarncke

17Neel Nanda

5orthonormal

3Neel Nanda

2deepthoughtlife

4Neel Nanda

1Thomas Dullien

1rpglover64

6Neel Nanda

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17 comments, sorted by Click to highlight new comments since: Today at 5:19 AM

**Self-Review**: After a while of being insecure about it, I'm now pretty fucking proud of this paper, and think it's one of the coolest pieces of research I've personally done. (I'm going to both review this post, and the subsequent paper). Though, as discussed below, I think people often overrate it.

**Impact** The main impact IMO is proving that mechanistic interpretability is actually possible, that we can take a trained neural network and reverse-engineer non-trivial and unexpected algorithms from it. In particular, I think by focusing on grokking I (semi-accidentally) did a great job of picking a problem that people cared about for non-interp reasons, where mech interp was unusually easy (as it was a small model, on a clean algorithmic task), and that I was able to find real insights about grokking as a direct result of doing the mechanistic interpretability. Real models are fucking complicated (and even this model has some details we didn't fully understand), but I feel great about the field having something that's genuinely detailed, rigorous and successfully reverse-engineered, and this seems an important proof of concept. IMO the other contributions are the specific algorithm I found, and the specific insights about how and why grokking happens. but in my opinion these are much less interesting.

**Field-Building** Another large amount of impact is that this was a major win for mech interp field-building. This is hard to measure, but some data:

- There are multiple papers I like that are substantially building on/informed by these results (A toy model of universality, the clock and the pizza, Feature emergence via margin maximization, Explaining grokking through circuit efficiency
- It's got >100 citations in less than a year (a decent chunk of these are semi-fake citations from this being used as a standard citation for 'mech interp exists as a field', so I care more about the "how many papers would not exist without this" metric)

- It went pretty viral on Twitter (1,000 likes, 300,000 views)
- This was the joint first mech interp paper at a top ML conference (ICLR 23 spotlight) and seemed to get a lot of interest at the conference.
- Anecdotally, I moderately often see people discussing this paper, or who know of me from this paper
- At least one of my MATS scholars says he got into the field because of this work.

**Is field-building good?** It's plausible to me that, on the margin, less alignment effort should be going into mech interp, and more should go into other agendas. But I'm still excited about mech interp field building, think this is a solid and high-value thing, and that field-building is often positive sum - people often have strong personal fits to different areas, and many people are drawn to it from non-alignment motivations. And though there's concern over capabilities externalities, my guess is that good interp work is strongly net good.

**Is it worth publishing in academic venues** Submitting this to ICLR was my first serious experience with peer review. I'm not sure what updates I've made re whether this was worth it. I think some, but probably <50% of the field-building benefit came from this, and that going Twitter viral was much more important for ensuring people became aware of the work. I think it made the work more legitimate-seeming, more citable, more respectable, etc. On an object level, it resulted in the writing, ideas and experiments becoming much better and clearer (though led to some of the more interesting speculation being relegated to appendix E :'( ) though this was largely due to /u/lawrencec 's skills. I definitely find peer review/conforming to academic conventions very tedious and irritating, and am very grateful to Lawrence for doing almost all of the work.

**Personal benefit** It's hard to measure, but I think this turned out to be substantially good for my career, reputation and influence. It's often been what people know me for, and I think has helped me snowball into other career successes.

**Ways it's overrated** As noted, I do think there's ways people overrate the results/ideas here, and that there's too much interest in the object level results (the Fourier Multiplication algorithm, and understanding grokking). Some thoughts:

- The specific techniques I used to understand the model are super specific to modular addition + Fourier stuff, and not very useful on real models (though I hold out hope that it'll be relevant to how language models do addition!)
- People often think grokking is a key insight about how language models learn, I think this can be misleading. Grokking is a fragile and transitionary state you get when your hyper-parameters are just right (a bit more or less data makes it memorise or generalise immediately) and requires a ton of overtraining on the same data gain and again. I think grokking gives some hints that individual circuits may be learned in sudden phase transitions (the quantization model of neural scaling), but we need much more evidence from real models on these questions. And something like "true reasoning" is plausibly a mix of many circuits, each with their own phase transition, rather than a thing that'll be suddenly grokked.
- People often underestimate the difficulty jump in interpreting real models (even a 1L language model) compared to the modular addition model, and get too excited about how easy it'll be
- People also get excited about more algorithmic interp work. I think this is largely played out, and focus on real models in my own work (and the work I supervise). I ultimately care about reverse-engineering AGI, and I think language models (even small ones) are decent proxies for this, while algorithmic problem models are not, unless you set up a really good one, that captures some property of real models that we care about. And I'm unconvinced there's much more marginal value in demonstrating that algorithmic mech interp is possible.

**[Edit Jan 19, 2023: I no longer think the below is accurate. My argument rests on an unstated assumption: that when weight decay kicks in, the counter-pressure against it is stronger for the 101th weight (the "bias/generalizer") than the other weights (the "memorizers") since the gradient is stronger in that direction. In fact, this mostly isn't true, for the same reason Adam(W) moved towards the **** solution to begin with ****before**** weight decay strongly kicked in: each dimension of the gradient is normalized relative to its typical magnitudes in the past. Hence the counter-pressure is the same on all coordinates.**

**A caveat: this assumes we're doing full batch AdamW, as opposed to randomized minibatches. In the latter case, the increased noise in the "memorizer" weights will in fact cause Adam to be less confident about those weights, and thus assign less magnitude to them. But this happens essentially right from the start, so it doesn't really explain grokking.**

**Here's an example of this, taking randomized minibatches of size 10 (out of 100 total) on each step, optimizing with AdamW (learning rate = 0.001, weight decay = 0.01). I show the first three "memorizer" weights (out of 100 total) plus the bias:**

**As you can see, it does place less magnitude on the memorizers due to the increased noise, but this happens right from the get-go; it never "groks".**

**If we do full batch AdamW, then the bias is treated indistinguishably:**

**For small weight decay settings and zero gradient noise, AdamW ****is**** doing something like finding the minimum-norm solution, but in ** **space, not **** space.]**

Here's a straightforward argument that phase changes are an artifact of AdamW and won't be seen with SGD (or SGD with momentum).

Suppose we have 101 weights all initialized to 0 in a linear model, and two possible ways to fit the training data:

The first is . (It sets the first 100 weights to 1, and the last one to 0.)

The second is . (It sets the first 100 weights to 0, and the last one to 1.)

(Any combination with will also fit the training data.)

Intuitively, the first solution memorizes the training data: we can imagine that each of the first 100 weights corresponds to storing the value of one of the 100 samples in our training set. The second solution is a simple, generalized algorithm for solving all instances from the underlying data distribution, whether in the training set or not.

has an norm which is ten times as large as . SGD, since it follows the gradient directly, will mostly move directly toward as it's the direction of steeper descent. It will ultimately converge on the minimum norm solution . (Momentum won't change this picture much, since it's just smearing out each SGD step over multiple updates, and each individual SGD step goes in the direction of steepest descent.)

AdamW, on the other hand, is basically the same as Adam at first, since weight decay doesn't do much when the weights are small. Since Adam is a scale-invariant, coordinate-wise adaptive learning rate algorithm, it will move at the same speed for each of the 101 coordinates in the direction which reduces loss, moving towards the solution , i.e. with heavy weight on the memorization solution. Weight decay will start to kick in a bit before this point, and over time AdamW will converge to (close to) the same minimum-norm solution as SGD. This is the phase transition from memorization to generalization.

Thanks, this is awesome! Especially cool that you were able to reverse-engineer the learned algorithm! And the theory/analysis seems great too.

[Just thinking aloud my confusion, not important] --Not sure I understand the difference between random walk hypothesis and lottery ticket inspired hypothesis. In both cases there's a phase transition caused by accelerating gradients once weak versions of all the components of the nonmodular circuit are in place. --The central piece of evidence you bring up is what exactly... that the effectiveness of the nonmodular circuit rises before the phase transition begins. I guess I'm a bit confused about how this works. If I imagine a circuit consisting of a modular memorized answer part and another part, the generalizing bit, that actually computes the answer... if the second part is really crappy/weak/barely there, I don't see how making it stronger is incentivised. Making it stronger means more weights so the regularization should push against it, UNLESS you can simultaneously delete or dampen weights from the memorized answer part, right? But because the generalizing part is so weak, if you delete or dampen weights from the memorized answer part, won't loss go up? Because the generalizing part isn't ready yet to take over the job of the memorized answers?

Thanks! I agree that they're pretty hard to distinguish, and evidence between them is fairly weak - it's hard to distinguish between a winning lottery ticket at initialisation vs one stumbled upon within the first 200 steps, say.

My favourite piece of evidence is [this video from Eric Michaud](https://twitter.com/ericjmichaud_/status/1559305105521926144) - we know that the first 2 principle components of the embedding form a circle at the *end *of training. But if we fix the axes at the end of training, and project the embedding at the *start *of training, it's pretty circle-y

Making it stronger means more weights so the regularization should push against it, UNLESS you can simultaneously delete or dampen weights from the memorized answer part, right?

I think this does happen (and is very surprising to me!). If you look at the excluded loss section, I ablate the model's ability to use one component of the generalising algorithm, in a way that shouldn't affect the memorising algorithm (much), and see the damage of this ablation rise smoothly over training. I hypothesise that it's dampening memorisation weights simulatenously, though haven't dug deep enough to be confident. Regardless, it clearly seems to be doing *some *kind of interpolation - I have a lot of progress measures that all (both qualitatively and quantitatively) show clear progress towards generalisation pre grokking.

You suggest that first the model quickly memorises a chunk of the data set, then (slowly) learns rules afterward to simplify itself. (You mention that noticing x + y = y + x could allow it to half its memorised library with no loss).

I am interested in another way this process could go. We could imagine that the early models are doing something like "Check for the input in my library of known results, if its not in there just guess something." So the model has two parts, [memorised] and [guess]. With some kind of fork to decide between them.

At first the guess is useless, and improves very slowly. So making sure the memorised information is correct and compact is the majority of the improvement. Eventually the memory saturates, and the only way to progress is refining the guess so the model does slightly-less awful in the cases it hasn't memorised. So here the guess steadily improves, but these improvements don't show up in the performance because the guess is still a fallback option that is rarely used. In this picture the phase change is presumably the point where the "guess" becomes so good the model "realises" it no longer needs its memorised library or the "if" tree.

These two pictures are quite different. In one information about the problem makes the library more efficient, leading eventually to an algorithm in the other the two strategies (memory, algorithm) are essentially independent and one eventually makes the other redundant.

Just the random thoughts of someone interested with no knowledge of the topic.

Interesting hypothesis, thanks!

My guess is that memorisation isn't really a discrete thing - it's not that the model has either memorised a data point or not, it's more that it's fitting a messed up curve to approximated all training data as well as it can. And more parameters means it memorises all data points a bit better, not that it has excellent loss on some data and terrible loss on others, and gradually the excellent set expands.

I haven't properly tested this though! And I'm using full batch training, which probably messes this up a fair bit.

Possibly also of interest: https://cprimozic.net/blog/reverse-engineering-a-small-neural-network/

You don't talk about human analogs of grokking, and that makes sense for a technical paper like this. Nonetheless, grokking also seems to happen in humans, and everybody has had "Aha!" moments before. Can you maybe comment a bit on the relation to human learning? It seems clear that human grokking is not a process that purely depends on the number of training samples seen but also on the availability of hypotheses. People grok faster if you provide them with symbolic descriptions of what goes on. What are your thoughts on the representation and transfer of the resulting structure, e.g., via language/token streams?

Hmm. So firstly, I don't think ML grokking and human grokking having the same name is *that *relevant - it could just be a vague analogy. And I definitely don't claim to understand neuroscience!

That said, I'd guess there's something relevant about phase changes? Internally, I know that I initially feel very confused, then have some intuition of 'I half see some structure but it's super fuzzy', and then eventually things magically click into place. And maybe there's some similar structure around how phase changes happen - useful explanations get reinforced, and as explanations become more useful they become reinforced much faster, leading to a feeling of 'clicking'

It seems less obvious to me that human grokking looks like 'stare at the same data points a bunch of times until things click'. It also seems plausible that there's some transfer learning going on - here I train models from scratch, but when I personally grok things it feels like I'm fitting the new problem into existing structure in my mind - maybe analogous to how induction heads get repurposed for few shot learning

This is honestly some of the most significant alignment work I've seen in recent years (for reasons I plan to post on shortly), thank you for going to all this length!

Typo: "*Thoughout* this process test loss remains *low* - even a partial memorising solution still performs extremely badly on unseen data!", 'low' should be 'high' (and 'throughout' is misspelled too).

I had an idea when reading it that I think is pretty interesting. You mention that both the grokking of a small amount of data repeated many times, and models trained on a great deal of data are highly general. Repeated data during training is also mentioned as a significant negative for large models. These are very much in tension.

My idea is this. Split the training data into two parts, one vastly larger than the other. First, train a model on a small amount of data many times in a way designed to make it grok the task, such as weight decay. Second, train it on the rest of the very large amount of data. Third, compare it to the a model trained on both parts without repeats. See how they compare. (I don't know if people have done this. I'm definitely a layman when it comes to such things.)

Models are often 'fine-tuned' on things later. You could see this as sort of the opposite, where we tune it for the task first, and then train it.

Repeated data during training is also mentioned as a significant negative for large models. These are very much in tension.

To be clear, the paper I cite on data quality focuses on how repeated data is bad for generalisation. From the model's perspective, the *only *thing it cares about is train loss (and maybe simplicity), and repeated data is great for train loss! The model doesn't care whether it generalises, only whether generalisation is a "more efficient" solution. Grokking happens when the amount of data is such that the model *marginally *prefers the correct solution, but there's no reason to expect that repeated data screwing over models is exactly the amount of data such that the correct solution is better.

Though the fact that larger models are messed up by fewer repeated data points is fascinating - I don't know if this is a problem with my hypothesis, or just a statement about the relative complexity of different circuits in larger vs smaller models.

Your experiment idea is interesting, I'm not sure what I'd expect to happen! I'd love to see someone try it, and am not aware of anyone who has (the paper I cite is vaguely similar - there they train the model on the repeated data and unrepeated data shuffled together, and compare it to a model trained on just the unrepeated data).

Though I do think that if this is a real task there wouldn't be an amount of data that leads to general grokking, rather than amount of data to grok varies heavily between different circuits.

Good stuff. A few thoughts:

1. Assuming a model has memorized the training data, and still have enough "spare capacity" to play lottery ticket hypothesis to find generalizing solutions to a subset of the memorized data, you'll eventually end up with a number of partial solutions that generalize to a subset of the memorized data (obviously assuming some form of regularization towards simplicity). So this may be where the "underparametrized" regime of ML of the past went wrong: That approach tried to force the model into generalization without memorization, but by being stingy with parameters, forced the model to first and foremost memorize -- there was no spare capacity to "play / experiment with possibly generalizing solutions" left. This then led to memorization-only models, to which researchers reacted by restricting parameters more ...

2. Occam's razor favors simpler models (for some definition of simplicity) over more complex models, given equal predictive power. The best definition of "model simplicity" that we have may in fact be Kolmogorov complexity of the weight matrices. This would mean that if we want a model to apply Occam's razor, we should see if we can use a measure of Kolmogorov complexity of the weights as regularization. The "best" approximation we currently have for Kolmogorov complexity is ... compression, which in itself is a prediction problem. So perhaps the way to encourage good generalization in models is to measure how good the *weights* can be predicted by another model (?). Apologies if this may sound like a crackpot idea.

3. It might be worth experimenting with shifting the regularization term during training, initially encouraging wide connectivity, and then shifting to either sparsity or low Kolmogorov complexity. There's an intriguing parallel to synaptic pruning in childhood.

Some potentially naive thoughts/questions:

- At a cursory level, this seems closely related to Deep Double Descent, but you don't mention it, which I find surprising; did I pattern-match in error?
- This also seems tangentially related to the single basin hypothesis

Idk, it *might* be related to double descent? I'm not that convinced.

Firstly, IMO, the most interesting part of deep double descent is the model size wise/data wise descent, which totally don't apply here.

They did also find epoch wise (different from data wise, because it's trained on the *same* data a bunch), which is more related, but looks like test loss going down, then going up again, then going down. You could argue that grokking has test loss going up, but since it starts at uniform test loss I think this doesn't count.

My guess is that the descent part of deep double descent illustrates some underlying competition between different circuits in the model, where some do memorisation and others do generalisation, and there's some similar competition and switching. Which is cool and interesting and somewhat related! And it totally wouldn't surprise me if there's some similar tension between hard to reach but simple and easy to reach but complex.

Re single basin, idk, I actually think it's a clear *disproof* of the single basin hypothesis (in this specific case - it could still easily be mostly true for other problems). Here, there's a solution to modular addition for each of the 56 frequencies! These solutions are *analogous*, but they are fairly far apart in model space, and definitely can't be bridged by just permuting the weights. (eg, the embedding picking up on vs is a totally different solution and set of weights, and would require significant non-linear shuffling around of parameters)

A significantly updated version of this work isnow on Arxivand was published as a spotlight paper at ICLR 2023aka, how the best way to do modular addition is with Discrete Fourier Transforms and trig identitiesIf you don't want to commit to a long post, check out theTweet thread summary## Introduction

Grokking is a recent phenomena discovered by OpenAI researchers, that in my opinion is one of the most fascinating mysteries in deep learning. That models trained on small algorithmic tasks like modular addition will initially memorise the training data, but after a long time will suddenly learn to generalise to unseen data.

This is a write-up of an independent research project I did into understanding grokking through the lens of mechanistic interpretability.

My most important claim is that grokking has a deep relationship to phase changes. Phase changes, ie a sudden change in the model's performance for some capability during training, are a general phenomena that occur when training models, that have also been observed in large models trained on non-toy tasks. For example, the sudden change in a transformer's capacity to do in-context learning when it forms induction heads. In this work examine several toy settings where a model trained to solve them exhibits a phase change in test loss, regardless of how much data it is trained on. I show that if a model is trained on these limited data with high regularisation, then that the model shows grokking.One of the core claims of mechanistic interpretability is that neural networks can be understood, that rather than being mysterious black boxes they learn interpretable algorithms which can be reverse engineered and comprehended. This work serves as a proof of concept of that, and that reverse engineering models is key to understanding them. I fully reverse engineer the inferred algorithm from a transformer that has grokked how to do modular addition (which somehow involves Discrete Fourier Transforms and trig identities?!), and use this as a concrete example to analyse what happens during training to understand what happened during grokking. I close with discussion and thoughts on the alignment relevance of these results.

This is accompanied by a paper in the form of a Colab notebook containing the code for this project, a lot of interactive graphics, and much more in-depth discussion and technical details. In this write-up I try to give a high-level conceptual overview of the claims and the most compelling results and evidence, I refer you to the notebook if you want the full technical details.

This write-up ends with a list of ideas for future directions of this research. I think this is a particularly exciting problem to start with if you want to get into mechanistic interpretability since it's concrete, only involves tiny models, and is easy to do in a Colab notebook. If you might want to work on some of these, please reach out! In particular, I'm looking to hire an intern/research assistant, and if you're excited about these future directions you might be a good fit.

## Key Claims

Grokking is really about phase changes:To exhibit grokking, we train a model on a problem that exhibits phase changes even when given infinite training data, and train it with regularisation and limited data. If we choose the amount of data such that the regularisation only marginally favours the generalising solution over the memorised solution, we see grokking.Intuition:Regularisation makes the model ultimately prefer the generalising solution to the memorised solution, but the phase change indicates that the generalising solution is "hard to reach" in some sense. The memorised solution is "easier to reach", and so is reached first. But due to the regularisation, the model still prefers the generalising solution to the memorised solution, and thus gets to the generalising solution eventually - the grokking result just shows that reaching the memorised solution first does not change this, and that there is a path in model space interpolating between the memorising and generalising solution.The algorithm learned to do modular addition can be fully reverse engineered.The algorithm is roughly:The generalising circuits used to do modular addition can be seen to smoothly evolve over the course of training, as the model interpolates between purely memorising and purely generalising## Phase Changes

Epistemic status: I feel confident in the empirical results, but the generalisation to non-toy settings is more speculative## Key Takeaways

Speculation:Phase changes occur in any circuit that involves multiple components composing.Intuitive explanation of grokking:Regularisation incentivises the model to be simpler, so the model prefers the generalising solution to the memorised solution. The generalising solution is "hard to reach" and the memorising solution is not, so the memorising solution is reached first. But the incentive to find the generalising solution is still there, so the underlying mechanism to induce the phase change is still going on while grokking - memorisation doesn't change thisSpeculation:Phase changes occur for any specific circuit in a model, and so occur all the time in larger models. Smooth loss curves are actually the average of many small phase changes## What Is A Phase Change?

By a phase change, I mean a reverse-S shaped curve

^{[1]}in the model's loss on some dataset, or the model's capacity for some specific capability. That is, the model initially has poor performance, this performance plateaus or slowly improves, and suddenly the performance accelerates and rapidly improves (ie, loss rapidly decreases), and eventually levels off.A particularly well-studied motivating example of this is Anthropic's study of

induction heads. Induction heads are a circuit in transformers used to predict repeated sequences of tokens. They search the context for previous copies of the current token, and then attend to the token immediatelyafterthat copy, and predict that that subsequent token will come next. Eg, if the 3 token surname`D|urs|ley`

is earlier in the context, and the model wants to predict what comes after`D`

, it will attend to`urs`

and predict that that comes next.The key fact about induction heads are that there is a fairly narrow band of training time where they undergo a phase change, and go from not existing/functional to being fully functional. This is particularly striking because induction heads are the main mechanism behind how Large Language Models do

in-context learning- using tokens from far back in the context to usefully predict the next token. This means that there is a clear phase change in a model's ability to do in-context learning, as shown below, and that this corresponds to the formation of a specific circuit via a phase change within the model.My goal here is to convey the intuition of what I mean by a phase change, rather than give a clear and objective definition. At this stage of research, I favour an informal "I know it when I see it" style definition, over something more formal and brittle.

## Empirical Observations

Motivation: Modular addition shows clear grokking on limited data^{[2]}, but given much more data it shows a phase change inbothtrain and test loss. This motivated the hypothesis that grokking could be observed any time we took a problem with a phase change and reduced the amount of data while regularising:In several toy algorithmic tasks I observe phase changes (in both test and train loss) when models are trained on sufficient data

^{[3]}.The tasks - see the Colab for more details

Modular addition mod 113(1L full transformer)5 digit addition(1L full transformer)`1|3|4|5|2|+|5|8|3|2|1|=|0|7|1|7|7|3`

Predicting Repeated Subsequences(2L attn-only transformer - task designed to need induction heads)`7 2 8 3 1 9 3`

- we take a uniform random sequence of tokens, randomly choose a subsequence to repeat, and train the model to predict the repeated tokens.8 3 1 99 2 5 ENDFinding the max element in a sequence(1L attn-only transformer - task designed to need skip trigrams)`START 0 4 7 2 4`

with149 7 2 5 3 ENDexactlyone entry ≥10, and the model is trained to output that entry after END. The solution is to learn 10 skip trigrams of the form`14 .. END 14`

## Grokking = Phase Changes + Regularisation + Limited Data

For all of the above tasks, we can induce grokking by adding regularisation (specifically weight decay) and limiting the amount of training data. Eg for 5 digit addition on 700 examples (see the notebook for the rest):

With enough data (eg single epoch training), the model generalises easily, and with sufficiently little data (eg a single data point), the model memorises. But there is a crossover point, and we identify grokking when training with slightly more data than the crossover point - analogous to the findings of Liu et al that grokking is an intermediate state between comprehension and memorisation. I found the crossover points (here, 700) by binary searching by hand. Intuitively, this is consistent with the idea that grokking occurs because of regularisation favouring the simpler solution. Memorisation complexity increases with the amount of data (approximately continuously) while generalising complexity does not, so there should must eventually be a crossover point.

This is particularly interesting, because these tasks are a non-trivial extension of the original grokking. Previous demonstrations of grokking had an extremely small universe of data and were trained on a substantial fraction of it, suggesting that the model may be doing some kind of clever interpolation. While here, the universe of data is much larger (eg 1010 possible pairs of 5 digit numbers), and model is trained on a tiny fraction of that, yet it still groks.

## Explaining Grokking

Epistemic status: The following two sections are fairly speculative - they're my current best explanation of my empirical findings, but are likely highly incomplete and could easily be totally wrong.## Speculation: Phase Changes Are Inherent to Composition

I recommend reading the section ofA Mathematical Framework for Transformer Circuits on Induction Headsto fully follow this sectionTo understand the link between phase changes and grokking, it's worth reflecting on why circuits form at all. A priori, this is pretty surprising! To see this, let's focus on the example of an induction circuit, and speculate on how it could be formed. The induction circuit is made up of two heads, a

previous token headand aninduction head, which interact by K-composition. Together, these heads significantly improve loss, but only in the context of the other head being there. Thus, naively, when we have neither head, there should be no gradient encouraging the formation of either head.At initialisation, we have neither head, and so gradient descent should never discover this circuit. Naively, we might predict that neural networks will only produce circuits analogous to linear regression, where each weight will clearly marginally improve performance as it continuously improves. And yet in practice, neural networks empirically do form sophisticated circuits like this, involving several parts interacting in non-trivial, algorithmic ways.

^{[4]}I see a few different possible explanations for this:

A lottery ticket hypothesis-inspired explanation:^{[5]}Initially, each layer of the network is the superposition of many different partial circuit components, and the output of each layer is the average of the output of each component. The full output of the network is the average of many different circuits. Some of these circuits are systematically useful to reducing loss, and most circuits aren't. SGD will reinforce the relevant circuits and suppress the useless circuits, so the circuits will gradually form.A random walk explanation:The network wanders randomly around the loss landscape, until it happens to get lucky and find a half-formed previous token head and induction head that somewhat compose. Once it has this, this half-formed circuit is useful for reducing loss and gradient descent can take over and make a complete circuit.An evolutionary explanation: There's a similar mystery for how organisms develop sophisticated machinery like the human eye, where each part is only useful in the context of other parts. The explanation I find most compelling is that we first developed one component that was somewhat useful on its own, eg a light-detecting membrane. This component was useful in its own right, and so was reinforced, and later components could develop that depend on it, eg the lenses in our eye.The evolutionary explanation is a natural hypothesis, but we can see from my toy tasks that it cannot be the whole story. In the repeated subsequence task, we have a sequence of uniform randomly generated tokens, apart from a repeated subsequence at an arbitrary location, eg

`7 2 8 3 1 9 3`

. This means that all pairs of tokens are independent,8 3 1 99 2 5 ENDapartfrom the pairs of equal tokens in the repeated subsequence. In particular, this means that a previous token head can never reduce loss for the current token - the previous token will always be independent of the next token. So a previous token head isonlyuseful in the context of an induction-like head that completes the circuit. Likewise, an induction head relies on K-composition with a previous token head, and so cannot be useful on its own. Yet the model eventually forms an induction circuit!^{[6]}A priori, the random walk story seems unlikely be sufficient on its own - an induction circuit is pretty complicated, and it likes represents a very small region in model space, and so seems unlikely to be stumbled upon by a random walk

^{[7]}. Thus my prediction is that the lottery ticket hypothesis is most of what's going on^{[8]}- an induction head will be useless without a previous token head, but may beslightlyuseful when composing with, say, a head that uniformly attends to prior tokens, since part of its output will include the previous token! I expect that all explanations are part of the picture though, eg this seems more plausible if the uniform head just so happens to attend abitmore to the previous token via a random walk, etc.Drawing this back to phase changes, the lottery ticket-style explanation suggests that we might expect to see phase changes as circuits form. Early on in circuit formation, each part of the circuit is very rough, so the effect on the loss of improving any individual component is weak, which means the gradients will be small. But as each component develops, each other component will become more useful, which means that all gradients will increase together in a non-linear way. So as the circuit becomes closer to completion we should expect an acceleration in the loss curve for this circuit, resulting in a phase change.

## An Intuitive Explanation of Grokking

With this explanation, we can now try to answer the question of why grokking occurs! To recap the problem setting, we are training our model on a problem with two possible solutions - the memorising algorithm and the generalising algorithm. We apply regularisation and choose a limited amount of data, such that the generalising solution is marginally simpler than the memorising solution

^{[9]}and so our training setup marginally prefers the generalising solution over the memorising solution. Naively, we expect the model to learn the generalising solution.But we are training our model on a problem whose solution involves multiple components interacting to form a complete circuit. So, early in training, the gradients incentivising each component of the generalising solution are weak, because they need the parts to all be formed and lined up properly. Memorisation, however, does not require several components to be lined up in careful and coordinated way

^{[10]}, so it does not have artificially weak gradients at the start. Thus, at the start, memorisation is incentivised more strongly than generalisation and the model memorises.So, why does the model

shiftfrom memorisation to generalisation? Eventually the training loss plateaus post-memorisation - loss is falling and total weights are rising so eventually the gradients towards lower loss (ie to memorise better) balances with the gradients towards lower weights (ie to be simpler) and cancel out. But they don'tperfectlycancel out. If there's a direction in model space that allows it to memorise more efficiently^{[11]}, then both gradients will encourage this direction. And the natural way to do this is by picking up on regularities in the data - eg, you can memorise modular addition twice as efficiently by recognising that x+y=y+x. This is the same process that leads the model to generalise in the infinite data case - it wants to pick up on patterns in the data.^{[12]}So the model is still incentivised to reach the generalising solution, just as in the infinite data case. But rather than moving from the randomly initialised model to the generalising model (as in the infinite data case), it interpolates between the memorising solution and the generalising solution. Throughout this process test loss remains high - even a partial memorising solution still performs extremely badly on unseen data! But once the generalising solution gets good enough, the incentive to simplify by deleting the remnants of the memorising solution dominates, and the model clears up the memorising solution, finally resulting in good test performance. Further, as in the infinite data case, the closer we get to the generalising solution the more the rate of change of the loss accelerates. So this final shift happens extremely abruptly - manifesting as the grokking phase change!

As a final bit of evidence, once the model has fully transitioned to the generalising solution it is now inherently simpler, and the point where the incentive to improve loss balances with the incentive to be simpler is marginally lower - we can observe in the graphs here that the model experiences a notable drop in train loss post grokking.

I later walk through this narrative and what it corresponds to in the modular addition case.

## Speculation: Phase Changes are Everywhere

My explanation above asserted that phase change are a natural thing to expect with the formation of specific circuits in models. If we buy the hypothesis that most things models do are built up out of many interpretable circuits, then shouldn't we expect to see phase changes everywhere whenever we train models, rather than smooth and convex curves?

My prediction is that yes, we should, and that in fact we do. But that larger models are made up of many circuits and, though each circuit may form in a phase change, the overall loss is made up out of the combination of many different capabilities (and thus many different circuits). And circuits of different complexity/importance likely form at different points in training. So the overall loss curve is actually the sum of many tiny phase changes at different points in training, and this overall looks smooth and convex. Regularities in loss curves like scaling laws may be downstream of statistical properties of the distribution of circuits, which become apparent at large scales. We directly observe the phase change-y ness from the loss curves in these simple toy problems because the problems are easy enough that only one/a few circuits are needed.

Some evidence for this hypothesis:

5 digit addition^{[13]}components - the loss on each of the 6 digits in the sum. When we do this, we observe separate phase changes in each digit, resulting in the many small non-convexities in the overall loss curve.^{[14]}Skip Trigrams`10 .. END 10`

,`11 .. END 11`

, etc. Each skip trigram shows a separate phase changeInduction Heads- Induction heads are the best studied example of a specific circuit through training, and there we see a clear phase change in LLMs up to a 13B transformer. Each head should be "as complex" as the others, so it makes sense that the all occur at approximately, but not exactly, the same time.^{[15]}Emergent phenomena- Emergent phenomena are a widely seen thing as language models scale up - they abruptly go from incapable to capable on a task such as addition. As argued by Jacob Steinhardt, More is Different in AI. It seems natural to extend this to a particular model during training, as we know that smaller models trained on more data can outcompete larger models on less dataAlphaZero Capabilities: One finding in DeepMind's AlphaZero Interpretability paper was that there is a phase change in the model's capabilities, where it learns to represent a lot of chess concepts around step 32,000.## Summary

I think this is highly suggestive evidence that there is a deep relationship between grokking and phase changes, and that grokking occurs when models with a phase change are trained with regularisation and limited data. I present some compelling (to me) explanations of what might be behind the phase change behaviour, and show how this model explains grokking and predicts several specific empirical observations about grokking. I don't claim to fully understand phase changes or grokking, but I do claim to have substantially reduced my confusion about grokking to my confusion about phase changes.

## Modular Addition

Epistemic status: I feel pretty confident that I have fully reverse engineered this network, and have enough different lines of evidence that I am confident in how it works. My explanation of how and why it develops during training is shakier.## Key Takeaways

## Model Details

In this section I dive deeply into one specific and well-checkpointed model trained to do modular addition. See model training code for more details, but here are the key points:

`x|y|=`

, where x,y are one-hot encoded inputs, and = is an extra token.^{[16]}This is a 1L transformer, with no LayerNorm and learned positional embeddings, trained with AdamW with weight decay 1, and full batch training on 30% of the data (the data is the 1132 pairs of numbers modp). The

## Overview of the Inferred Algorithm

The key feature of the algorithm is calculating cos(w(x+y)),sin(w(x+y)) with w=2πpk - this is a function of x+y and be mapped to x+y, and because cos(wx) has period pk we get the (modp) part for free.

More concretely:

There are a few adjustments to implement this algorithm in a neural network:

^{[17]}(different frequencies correspond to different clusters of neurons, different subspaces of the residual stream, and sometimes different attention heads)## Background on Discrete Fourier Transforms

A key technique in all that follows is Discrete Fourier Transforms (DFT). I give a more in-depth explainer in the colab, but here's a rough outline - I expect this requires familiarity with linear algebra to really get your head around. The key motivating observation is that most activations inside the network are periodic and so techniques designed to represent periodic functions nicely are key. Eg the attention patterns:

In Rp, we have a standard basis of the p unit vectors. But we can also take a basis of p cosine and sine waves, F∈Rp×p, where F0=(1,1,...,1) is the constant vector, and F2k−1=cos(2πpkx) and F2k=sin(2πpkx) are the cosine and sine wave of frequency w=2πpk (henceforth referred to as frequency w=k and represented as coskx for brevity) for k=1,...,p−12. Every pair of waves has dot product zero, unless they're the same wave (ie it's an orthogonal basis). If we normalise these rows, we get an orthonormal basis of cosine and sine waves (so F−1=FT). We refer to these normalised waves as

Fourier Componentsand this overall basis as the1D Fourier Basis.We can apply a change of basis to the 1D input space Rp to F, and this turns out to be a much more natural way to represent the input space for this problem, as the network learns to operate in terms of sine and cosine waves. Eg, the fourth column of WEFT is the direction corresponding to sin2x in the embedding for WE. If we apply this change of basis to both the input space for x and for y we apply a 2D DFT, and can represent any function as the linear combination of terms of the form sinw1xcosw2y (or cos(w1x)cos(w2y), Const∗cos(w2y), etc). This is just a change of basis on Rp×p=Rp2, and terms of the form sinw1xcosw2y (ie the outer product of each pair of rows in F) form an orthogonal basis of p2 vectors (henceforth referred to as the

2D Fourier Basis).Importantly, this is just a change of basis! If we choose any single activation in the network, this is a real valued function on pairs of inputs x,y∈Rp×p, and so is equivalent to specifying a p2 dimensional vector. And we can apply an arbitrary change of basis to this vector. So we can

alwayswrite it as a linear combination of terms in the 2D Fourier Basis. And any vector of activations is a linear combination of 2D Fourier terms times fixed vectors in activation space. If a function is periodic, this means that it issparsein the 1D or 2D Fourier Basis, andthisis what tells us about the structure of the algorithm and weights of the network.## Reverse Engineering the Algorithm

Here, I present a case for how I was able to reverse engineer the algorithm from the weights. See the Colab and appendices (attention and neuron) for full details, my goal in this section is to roughly sketch what's going on and why I'm confident that this is what's going on.

## Calculating waves cos(wx),sin(wx),cos(wy),sin(wy)

Relevant notebook sectionTheory:Naively, this seems like the hard part, but is actually extremely easy. The key is that we just need to learn the discretised wave on x∈[0,1,...,p−1], not for arbitrary x∈R. x is input into the network as a one-hot encoded vector, and the multiplied by a learned matrix WE. We can in fact learnanyfunction f:[0,1,...,p−1]→R^{[18]}Conveniently, F, the matrix of waves cos(wx),sin(wx), is an orthonormal basis. So WEFT will recover the direction of the embedding corresponding to each wave Const,cosx,sinx,cos2x,... - in other ways, extracting cos(wx),sin(wx) is just a rotation of the input space.

Evidence:We can use the norm of the embedding of each wave to get an indicator of how much the network "cares" about each wave^{[19]}, and when we do this we see that the plot is extremely sparse. The model has decided to throw away all but a few frequencies^{[20]}. This is very strong evidence that the model is working in the Fourier Basis - we would expect to see a basically uniform plot if this was not a privileged basis.## Calculating 2D products of waves cos(wx)cos(wy),cos(wx)sin(wy),sin(wx)cos(wy),sin(wx)sin(wy)

Relevant notebook sectionTheory: A good mental model for neural networks is that they arereallygood at matrix multiplication and addition, and anything else takes a lot of effort^{[21]}. As so here! As we saw above, creating cos(wx),sin(wx) is just a rotation, and the later rearranging and map to the logits is another linear map, but multiplying the terms together is hard and non-linear.There are three non-linear operations in a 1L transformer - the attention softmax, the element-wise product of attention and the value vectors, and the ReLU activations in the MLP layer. Here, the model uses both ReLU activations and element-wise products with attention to multiply terms

^{[22]}.The neurons form 5

^{[23]}distinct clusters for each frequency, and each neuron in the cluster for frequency w has its activation as a linear combination of 1,cos(wx),sin(wx),cos(wy),sin(wy),cos(wx)cos(wy),cos(wx)sin(wy),sin(wx)cos(wy),sin(wx)sin(wy).^{[24]}Note that, as explained above, the neuron activation inanynetwork can be represented as a linear combination of products of Fourier terms in x and Fourier terms in y (because they form a basis of Rp×p). The surprising fact is that this representation issparse! This can be visually seen as neuron activations being periodic:Evidence:The details of how the terms are multiplied together are highly convoluted^{[25]}, and I leave them to the Colab notebook appendices. But the neurons do in fact have the structure I described, and this can be directly observed by looking at their values. And thus, by this point in the network it has computed the product terms.For example, the activations for neuron 0 (as plotted above) are approximately 109−39(cos42x+cos42y)−76(sin42x+sin42y)+36(cos42xsin42y+sin42xcos42y)−10cos42xcos42y+38sin42xsin42y (these coefficients can be calculated by mapping the neuron activation into the 2D Fourier Basis). This approximation explains >90% of the variance in this neuron

^{[26]}. We can plot this visually with the following heatmap:Zooming out, we can apply a 2D DFT to all neuron activations, ie writing all of the neuron activations as a linear combinations of terms of the form cos42xcos42y times vectors, and plotting the norm of each vector of coefficients. Heuristically, this is telling us what terms are represented by the network at the output of the neurons. We see that the non-trivial terms are in the top row (of the form coswx,sinwx) or the left column (of the form coswy,sinwy) or in a cell of 2D cells along the diagonal (of the form cos(wx)cos(wy),cos(wx)sin(wy),sin(wx)cos(wy),sin(wx)sin(wy) - notably, a product term where both terms have the same frequency).

## Calculating cos(w(x+y)),sin(w(x+y)) and calculating logits

Relevant notebook sectionTheory: The operations mapping cos(w(x+y))=cos(wx)cos(wy)−sin(wx)sin(wy) and sin(w(x+y))=cos(wx)sin(wy)+sin(wx)cos(wy) are linear, and the operations mapping this to cos(w(x+y−z))=cos(w(x+y))cos(wz)+sin(w(x+y))sin(wz) are also linear. So their composition is linear, and can be represented by a single matrix multiplication. The neurons are mapped to the logits by L=WUWoutN, and so the effective weight matrix Wlogit=WUWout must represent both of these operations (if my hypothesis is correct). Note that Wlogit is a p×dmlp matrix, mapping from MLP-space to the output space.Evidence: We draw upon several different lines of evidence here.We show that the terms cos(w(x+y)),sin(w(x+y)) are computed as follows: We repeat the above analysis to find terms represented by the neurons on the logits, we find that the terms in the top row and left column cancel out. This leaves just diagonal terms, corresponding to products of waves of the same frequency in x and y, exactly the terms we need. We also see that the 2x2 blocks are uniform, showing that cos(w(x+y)) and sin(w(x+y)) have the same coefficient. Further analysis shows that everything other than cos(w(x+y)),sin(w(x+y)) for these 5 frequencies is essentially zero.

We now show that the output logits produce cos(w(x+y−z))=cos(w(x+y))cos(wz)+sin(w(x+y))sin(wz) for each of the 5 represented frequencies (where z is the term corresponding to the output logits). The neurons form clusters for each frequency, and when we plot the columns of Wlogit corresponding to those frequencies, and apply a 1D DFT to the output space of Wlogit, we see that the only non-trivial terms are cos(wz),sin(wz) - ie the output logits coming from these neuron clusters is a linear combination of cos(wz),sin(wz).

We can more directly verify this by writing approximating the output logits as a sum of ∑w∈[14,35,41,42,52]Awcos(w(x+y−z)) and fitting the coefficients Aw. When we do this, the resulting approximated logits explains 95% of the variance in the original logits. If we evaluate loss on this approximation to the logits, we actually see a significant

dropin loss, from 2∗10−7 to 4.7∗10−8## Evolution of Circuits During Training

Note:For this section in particular, I recommend referring to the Colab! That contains a bunch of interactive graphics that I can't include here, where we can observe the development of circuits during training.Now that we understand what the model is doing during training on a mechanistic level, we can directly observe the development of these circuits during training. The key observation is that the circuits develop smoothly, and make clear and systematic progress towards the generalising circuit well before there is notable test performance. For example, the graph of the norms of the embedding of different waves becomes sparse. In particular, this disproves the natural hypothesis that grokking is the result of a random walk on the manifold of models that perform optimally.

I will now give and explain several graphs of metrics during training, and use them to support my above narrative of what happens during grokking in this specific case:

Excluded loss: Intuitively, this metric tracks how much the model's performance on thetrainingdata comes from the generalising algorithm vs the memorisation algorithm, by ablating the model's ability to use a specific frequency w. This serves as aprogress measure, quantitatively tracking the model's progress towards the correct algorithm.Trig loss: Another metric is to take the output logits, andonlyallow them to use the directions corresponding to cos(w(x+y)),sin(w(x+y)) for the 5 relevant frequencies. Intuitively, this represents the model's performance with the generalising algorithm preserved but the memorising algorithm removed.We can now use these metrics to analyse the separate stages of training:

Memorisation(Epoch 0-1K): The model memorises. We see a dramatic drop in train loss, and a rise in test loss.Interpolation(Epoch 1K-9K): The model is interpolating between memorisation and generalisation.caninterpolate between two circuits while retaining good performance!nothappen in the first stage shows that the incentive for small weights is too weak and has little effectGeneralisation(Epoch 8K-12K): We see a phase change in trig loss. Trig loss represents "model performance if we delete the memorisation circuit", so this shows that the generalising circuit rapidly develops in a phase change.Cleaning Up Noise(Epoch 9K-13K): Somewhat in parallel with the above phase change in trig loss, there is a slightly delayed phase change in test loss. The slight delay indicates that good test loss performance requires both a strong generalising circuit,andthe removal of the memorising circuit.Stability(Epoch 13K-end)^{[27]}: The model has now transitioned to using the generalising circuit and all our metrics settle downsomeremnants of the memorisation circuit, as train loss is still less than test loss. This must come from directions other than cos(w(x+y)),sin(w(x+y)) in logit space, as trig loss is identical on the train and test data.## Discussion

## Alignment Relevance

Ultimately, my goal is to do research that is relevant to aligning future powerful AI systems. Here I've studied tiny models

(Note - the true reason I did this project was that I was on holiday, and got extremely nerd-sniped, so this is all post-hoc reasoning!)

Emergent capabilities:isdirectly relevant to large models is the indication that phase changes are everywhere. If most circuits emerge as phase changes during training, this suggests we should expect emergent capabilities to be a frequent occurence. In particular, this may occur for particular dangerous capabilities, such as deception or situational awarenessA laboratory for training dynamics + generalisation: There are many open questions about exactly why neural networks work, why they learn to generalise to unseen data, what kinds of generalising algorithms they learn, and how this can be influenced. These are extremely hard questions to study, and I don't feel that I've understood them as a result of this work. But I think the modular addition transformer may be a good laboratory to examine these questions, as it's a concrete, small model that can be easily trained and whose circuits can be clearly inspected. Further, understanding these questions is a key question for alignment - aligning systems will likely require predicting when they would learn to be deceptive vs aligned and how to influence them.Importance of interpretability: A high-level motivation behind this work is a belief that mechanistic interpretability is key to understand how neural networks work. They seem to learn interpretable circuits, and these determine both the model's capabilities, andhowthose capabilities are implemented. It seems very difficult to get traction on what's really going on in models without engaging with these circuits - analogous to doing chemistry without discovering the periodic table. I take this work as a proof of concept of that - I took the weird and confusing phenomena of grokking, and feel that I have significantly de-mystified it by fully reverse engineering the underlying circuits.^{[28]}algorithm for doing modular addition via reverse engineering.## Limitations

There are several limitations to this work and how confidently I can generalise from it to a general understanding of deep learning/grokking, especially the larger models that will be alignment relevant. In particular:

alwayslearn more circuits to achieve marginally better task performance, and so need to use their parameters efficiently. This may act as a form of implicit regularisation.^{[29]}## Future Directions

I see a lot of future directions building on this work. If you'd be interested in working on any of these, please reach out! I'd love to chat and possibly advise or collaborate on a project. I'm also currently looking to hire an intern/research assistant to help me work on mechanistically interpretability projects (including but not limited to these ideas), and if you feel excited about any these directions you may be a good fit.

Investigating the 'phase changes are everywhere' hypothesis.Investigating the 'phase changes are inherent to composition' hypothesisNote:Once a model has memorised, loss can be around 1e-7. SGD gradients scale with loss while Adam is normalised, so you'll need to increase your learning rate a lot.Loose threadsin this project^{[30]}Training dynamics.Note that a 2L MLP can also grok modular addition, and may be easier to study.^{[31]}? Can we accelerate the grokking point? Can we disincentivise it? Can we incentivise or disincentivise specific frequencies?How general is this algorithm?Can we find Fourier-style algorithms in LLMs? Especially when they learn to do addition`28|79|23|598|23|45|987|249|234|0000|23|47|89|03|00000|700|9|02|10`

. Sadly, the only tokenizer I know of like this is PaLM's.mostlycreates 2 digit tokens, so I'd start by trying to use a linear probe to extract cos(w(z)),sin(w(z)) in base 100 from the model for each frequency, where z is the correct output digit (or possible the output if you incorrectly carry a 1)Getting into the field: I think this problem is notable for being fairly simple and involving small models that it's tractable to completely understand. If you're interested in getting into mechanistic interpretability and are new to the field, I think this may be a great place to start! As a concrete first project, I recommend training your own transformer for modular subtraction, figuring out the analogous algorithm it should be using, and see if you can replicate my analysis to reverse engineer that.## Meta

## Notable Observations

Some scattered thoughts that feel exciting to me, but didn't naturally fit into this write-up:

Slingshot mechanism: (see colab) The Slingshot Mechanism was a fascinating paper from Thilak et al, that found that grokking seemed tied to 'slingshots', dramatic loss spikes, that it was hypothesised flung the model to different regions in the loss landscape.^{[32]}, and further that the slingshots are the result of a float32 precision error in`log_softmax`

.`log_softmax`

. If I keep my training code exactly the same but remove this casting, my loss curve instead looks like this monstrosity:Explanation:Concretely, the smallest non-zero value that`log_softmax`

can output in float32 is`1.19e-7`

. So once the model has memorised sufficiently well that loss approaches the scale of`1e-7`

, some data points will have loss rounding to zero. This also rounds the gradients of their loss to zero, equivalent to them not being in the training set. When a model memorises data, it wants to repurpose all other parameters and set unseen data points to extreme values, and this precision error means this includes some training data points. This introduces a dodgy gradient, and the EWMA of Adam ensures it persists for long enough for a significant loss spike`log_softmax(x)`

is equal to`log(1+x)`

, and this is approximately`x`

.`1.19e-7`

is the smallest value such that`1+x!=1`

in float32.Weight Decay: Naively, for a fixed training dataset size, the model should only grok for sufficiently high weight decay. But this is not actually true! Lower weight decay just delays grokking rather than stopping it.Intuition:The model will have an incentive to switch to the generalising circuit so long as it has lower total weight than the memorising circuit. Importantly, this holds foranynon-zero weight decay. Smaller weight decay just leads to a smaller incentiveExplaining circle plots ofWE: A notable finding of Liu et al and the original grokking paper is that the embedding matrix of modular addition can be plotted as a circle, by the first two principal components or t-SNE respectively. Now that we understand the underlying circuits, we can see that this clearly follows from the fact that the embedding matrix has orthogonal rows for sin(wx),cos(wx) - these dimensionality reduction techniques are just finding two directions of the same frequency, and observing that a scatter plot of sin(wx),cos(wx) is a circle2L MLPs can grok modular addition: A surprising (to me) result from Liu et al is that a 2L MLP can grok modular addition^{[33]}. I replicated this result, and found that it learned the same algorithm (here, purely using the ReLU/GeLU activations to multiply the waves in x and y). The neurons formed a cluster for every single frequency, with 3-7 neurons per cluster (width 256), and the input and output weights were sparse in the 1D Fourier basis.## Acknowledgements

This work was done as independent research after leaving Anthropic, but benefitted greatly from my work with the Anthropic interpretability team and skills gained, especially from an extremely generous amount of mentorship from Chris Olah. It relies heavily on the interpretability techniques and framework developed by the team in A Mathematical Framework for Transformer Circuits, and a lot of the analysis was inspired by the induction head bump result.

Thanks to Noa Nabeshima for pair programming with me on an initial replication of grokking, and to Vlad Mikulik for pair programming with me on the grokked induction heads experiment.

Thanks to Jacob Hilton, Alex Ray, Xander Davies, Lauro Langosco, Kevin Wang, Nicholas Turner, Rohin Shah, Vlad Mikulik, Janos Kramar, Johannes Treutlein, Arthur Conmy, Noa Nabeshima, Eric Michaud, Tao Lin, John Wentworth, Jeff Wu, David Bau, Martin Wattenberg, Nick Cammarata, Sid Black, Michela Paganini, David Lindner, Zac Kenton, Michela Paganini, Vikrant Varma, Evan Hubinger (and likely many others) for helpful clarifying discussions about this research, feedback, and helping me identify my many ill-formed explanations!

Generously supported by the FTX regrantor program

## Author Contributions

Tom Lieberum significantly contributed to the early stages of this project - replicating grokking for modular subtraction, discovering it was possible in a 1L transformer with no LayerNorm, and observing the strongly periodic behaviour of activations and weights

Neel Nanda led the rest of this project - fully reverse engineering the modular addition transformer, analysing it during training, and discovering and analysing the link between grokking and phase changes. He wrote this write-up, and it is written from his perspective.

## Feedback

I'd love to hear feedback on this work - parts you find compelling, parts you're skeptical of, parts I explained poorly, cases for why Colab notebooks are a terrible format for papers, etc. You can comment here, or reach me at neelnanda27@gmail.com.

## Citation Info

Please cite this work as:

^{^}In the ideal case this looks flat, then drops abruptly, then is flat again. In practice, I'm happy to include any striking non-convexities in a curve.

^{^}Ie a divergence in train and test loss

^{^}Where possible, I train the models on infinite data (ie, the data is randomly generated each time, and there are enough possible data points that nothing gets repeated) - in this case I do not distinguish between train and test loss, as train loss is always on unseen data.

^{^}This is even more mysterious when you consider how each head has both an attention (QK) circuit and a value (OV) circuit. The previous token head's QK circuit needs to compose with the positional embeddings and its OV circuit needs to compose with the embedding's output. The induction head's QK circuit needs to compose with the previous token head's output on the K-side, and the embedding on the Q-side,

andthe OV circuit needs to compose with the embedding and unembedding to be the identity. This is a lot of synchronised and non-trivial interaction! But we know that real networks form circuits with as many moving parts as this, or more, eg curve detecting circuits.^{^}Eric Michaud gives some evidence for this by finding that, if we project the model's initialised embedding onto the principal components of the final embedding, we still see a circle-like thing - suggesting that the model reinforced directions in embedding space that luckily had good properties.

^{^}I have not verified that the model actually forms an induction circuit of the form described in this particular task. But the structure of attention means it must be doing

somekind of composition, because a single attention head cannot attend to a previous token based on its neighbours^{^}I later show in the case of modular addition that it is definitively not a random walk, as we can see clear progress towards the true answer before grokking

^{^}Though I'm sure I'm missing other explanations!

^{^}This is a more complex statement than it looks, since each solution can always become more complex to result in lower loss. Eg doubling WU will always decrease loss if there's 100% accuracy. I operationalise this as 'which solution is simpler if we set them both to achieve the same level of loss'

^{^}I am assuming this fact about memorisation as part of this story. I don't understand how memorisation is implemented so I cannot be confident, but this seems plausible given that, eg a logistic/linear regression model with enough parameters can memorise random data

^{^}ie, more simply/with lower total weights

^{^}In some sense, single epoch training is the limiting case of this. The model tries to memorise every data point it sees, but there must eventually be a cap on how much data it can memorise, so it

mustpick up on patterns. We never run the model on the same data point twice, so we don't notice this attempt at memorisation. Plausibly, models that never learn to generalise keep trying to memorise recent training data and forget about old data.^{^}The 0th digit is the leading digit that's 0 or 1

^{^}This is weird! Token 5 (the final digit) should be easiest because it never requires you to carry a 1 - it's just addition mod 10

^{^}Under this hypothesis, I would predict that soft induction heads like translation appear after the phase change, as they seem likely more complex

^{^}The training run was up to 50K epochs, but there's a mysterious bonus tiny phase change at epoch 43K that I don't yet understand, and so I snap the model before then, as I don't think that shift is central to understand the first grok.

^{^}This is useful because, though z=x+y(modp) is an argmax of cos(w(x+y−z)), the difference between it and the second largest value is tiny. While if we take the sum of cos(w(x+y−z)) for several different values of w, the waves constructively interfere at z=x+y(modp), and destructively interfere everywhere else, significantly increasing the difference.

^{^}By having a column of WE be (f(0),f(1),...,f(p−1)) - the dot product (ie the first coordinate of the embedding) is f(x). In fact, so long as WE is invertible, we can recover any function f(x) by projecting onto the direction f(x)W−1E

^{^}Remember that we're using high weight decay, so the model will set any weights it's not using to (almost) zero

^{^}The frequencies here are 14,31,35,41,42,52 - as far as I can tell, the specific frequencies and the number of frequencies are arbitrary, and vary between random seeds

^{^}But is also extremely necessary! Eg how activation functions take an MLP from linear regression to being able to represent any function.

^{^}The attention pattern multiplication is the obvious way to do it - I found the use of ReLUs very surprising! But it turns out that ReLU(0.5+cos(kx)+cos(ky)) is approx 0.71+0.695cos(kx)+0.695cos(ky)+0.43cos(kx)cos(ky) (more details)

^{^}The model calculates 6 frequencies with the embedding, but only 5 with the neurons - I'm not sure why.

^{^}There is actually a sixth cluster not aligned with any particular frequency - neurons in this cluster always fire, and thus ReLU is the identity. I'm not entirely sure why this is useful to the network

^{^}In particular, understanding ReLU in the Fourier basis takes it from a nice basis aligned operation to the operation "rotate out of the Fourier basis into the standard basis, take the elementwise ReLU, and then rotate back into the Fourier basis"

^{^}We can validate that the remaining 10% is just noise by ablating - replacing each neuron by this approximation (ie removing all terms of all other frequencies) leaves loss unchanged.

^{^}This isn't quite true - at epoch 43K, there is a tiny phase change which can be seen if you zoom in on the trig loss graph (trig loss and train loss fall, test loss rises). I haven't investigated this properly, but the model goes from never attending to the

`=`

token to significantly attending to it. Presumably there's some further circuit development here?^{^}Well, new to me - ancedotally, a friend of mine came up with the algorithm independently when exploring grokking

^{^}Though skip trigrams are extremely easy to interpret, and mostly match what I'd predict, I just haven't written it up.

^{^}I've looked at several models, but not exhaustively

^{^}Note that excluded loss specifically is weird, because we fit the coefficients of cos(w(x+y)) to the logits across

allof the data, so it's technically also a function of the test data. Probably linear regression on just the train logits would suffice though.^{^}Though plausibly helps by adding regularisation - generalising solution may be more stable to slingshots than memorised ones?

^{^}Input format is the sum of a one-hot encoding of x and of y in R113. If x=y then the vector has a single two in it.