Not all causes are equally cost-effective. In fact, the distribution of cost-effectiveness is heavy-tailed: some causes are thousands of times more cost-effective than others.
Such variation in cost-effectiveness in part follows from the existence of immense global inequality, which implies that many people in the world suffer from problems that could be solved with only small amounts of money. Consider, for instance, a developing world charity that spends $20 per person on a surgery to prevent blindness, compared to a charity that spends $40,000 per person to provide guide dogs to blind people in the United States. But even charities that think globally are far from equal in their abilities to turn the donations they receive into real improvements in people’s lives.
Within a given focus area, we can understand variation in cost-effectiveness as arising from both underlying variation in the impact of the interventions that charities carry out and variation in how much charities spend to carry them out. For instance, the cost-effectiveness of a charity that combats malaria will depend both on whether it distributes medication or bednets and on how much wasteful spending it engages in. People in the effective altruism community tend to believe that the former source of variation is more significant than the latter.
One general finding is that, in the long run, the cost-effectiveness of additional donations to many charities will diminish as the amount they have already received grows. Because of this phenomenon, we would expect the cost-effectiveness of charities to become more and more equal in a world where donations were based on cost-effectiveness.