I attended an Effective Altruism club today where someone had this to say about longtermism.

I have an intuitive feeling that ethical arguments about small probabilities of helping out extremely large numbers (like ) of people are flawed, but I can't construct a good argument for why this is.

The flaw is uncertainty.

In the early 20th century, many intellectuals were worried about population control. The math was simple. People reproduce at an exponential rate. The amount of food we can create is finite. Population growth will eventually outstrip production. Humanity will starve unless population control is implemented by governments.

What actually happened was as surprising as it was counterintuitive. People in rich, industrial countries with access to birth control voluntarily restricted the number of kids we have. Birthrates fell below replacement-level fertility. This process is called the demographic transition.

We now know that if you want to reduce population growth, the best way to do so is to make everyone rich and then provide free birth control. The side effects of this are mostly beneficial too.

China didn't know about the demographic transition when they implemented the one-child policy (一孩政策). The one-child policy wasn't just a human rights disaster involving tens of thousands of forced abortions for the greater good. It was totally unnecessary. The one-child policy was implemented at a time when China was rapidly industrializing. The Chinese birthrate would have naturally dropped below replacement level without government intervention. Chinese birthrates are still below replacement-level fertility even now that the one-child policy has been lifted. China didn't just pay a huge cost to get zero benefit. They paid a huge cost to gain negative benefit. Their age pyramid and sex ratios are extra messed up now. This is the opposite of what effective population control should have accomplished.

China utterly failed to predict its own demographic transition even though demographic changes on time horizons of a few decades are an unusually easy trend to predict. The UN makes extremely precise predictions on population growth. Most trends are harder to predict than population growth. If you're making ethical decisions involving the distant future then you need to make predictions about the distant future. Predictions about the distant future necessarily involve high uncertainty.

In theory, a 10% chance of helping 10 people equals a 0.001% chance of helping out 100,000 people. In practice, they are very different because of uncertainty. In the 10% situation, a 0.1% uncertainty is ignorably small. In the 0.001% situation, a 0.1% uncertainty dominates the calculation. You have a 0.051% chance of doing good and a 0.049% chance of doing harm once uncertainty is factored in. It's statistical malpractice to even write the probabilities as "0.051%" and "0.049%". They both round to 0.05%.

Is it worth acting when you're comparing a 0.051% chance of doing good to a 0.049% chance of doing harm? Maybe, but it's far from a clean argument. Primum non nocere (first, do no harm) matters too. When the success probability of an altruistic action is lower than my baseline uncertainty about reality itself, I let epistemic humility take over by prioritizing more proximate objectives.