Two of the stranger trends of recent decades have been the Flynn effect (the tendency for IQ to rise over time) and the decline of violence: yes, including terrorism*. A new paper (pdf) by Eugenio Proto and colleagues suggests that these two trends might be related.

They show that in repeated prisoners' dilemma games, people with higher IQs are more likely to cooperate with each other and less likely to defect, with the result that they end up with higher pay-offs.

This is not because they have an innate tendency to cooperate more: in the early rounds of the games, there was no difference in the strategies chosen by low or high-IQ subjects. Nor was it because IQ is correlated with other personality: conscientiousness, they found, had no effect upon whether subjects cooperated or defected.

Instead, it seems that high-IQ subjects learned quicker that reciprocation and cooperation were the best strategies. They were more likely to play tit-for-tat and less likely to always defect; this might or might not be related to the tendency for high-IQ people to have lower time discount rates.

It's easy to see a possible link here with the decline of violence. Violence is a non-cooperative strategy and so would decline as IQ rises.

This might be reinforced by another mechanism - peer effects. As violence declines, it becomes aberrant rather than normal and so carries a stigma, which disincentivizes violence**.

This might also explain why there tends to be more violence in poor areas. It's not just because the poor have less to lose from criminal acts, but because the poor tend to have lower intelligence; this is not (just?) because low intelligence causes poverty, but because poverty reduces (pdf) intelligence***.

I don't offer this as the only, or even best, reason for the decline of violence; it's probably just one of many. And I certainly don't want to join those rather rum coves who think that IQ is enormously important. Instead, I say it for another reason. Perhaps this reminds us that social change happens unintentionally and even without our knowing it at the time. To paraphrase a noted purveyor of violence, progress is what happens when you are busy making other plans.

* Despite Diego Costa's efforts, scenes like these are rarer today.

** This first struck me at the 1999 Arsenal-Manyoo semi-final. After the match, a few people ran onto the pitch to fight. The response of most other fans was to laugh derisively at them as their bald pates glistened under the floodlights and their haymaker punches missed by miles.

*** I'm thinking here of low-level street scraps; it is not (pdf) the case that poverty causes terrorism.