How co-operation evolved in human societies is a long-standing scientific puzzle. To solve it, we need to explain both how co-operation arose in the first place and how it has been maintained over time. In a new paper led by Nadiah, we show using a mathematical model that two key ingredients can explain the puzzle: high homophily in early human societies, and non-linear pay-offs. Homophily refers to the tendency to interact with genetically related others. Non-linear pay-offs mean that the benefits of an activity such as a hunt may increase sharply once the number of hunters exceeds a certain threshold. The emerging chronological narrative is that human co-operative behaviour arose because ancient human groups were small and comprised mostly family members, and that it has been maintained over subsequent millennia because of non-linear pay-offs, even as human groups have become very large and homophily has dropped. Our mathematical model takes these ideas, which had previously been expressed only verbally, and formalises them and makes them rigorous.
A major challenge in our modelling approach was computing higher-order genetic association between individuals, and to overcome this we used a mathematical framework developed several years ago by our collaborator Hisashi Ohtsuki from the School of Advanced Sciences in Japan. Our new paper is now published in Scientific Reports: