How altruistic behaviour emerges has puzzled evolutionary biologists for decades. From the point of view of survivial of the fittest, the unselfish concern for the welfare of others seems inexplicable. Surely any organism should always act selfishly if it were truly intent on saving its own bacon.

One explanation is that altruistic acts, although seemingly unselfish, actually benefit those who perform them but in indirect ways. The idea is that unselfish acts are repeated. So those who have been helped go on to help other individuals, ensuring that this behaviour spreads through a group, a phenomenon known as upstream reciprocity.

Eventually, the individual that triggered the altruistic behaviour will be on the receiving end of least one unselfish act, ensuring that, at the very least, he or she doesn’t lose out. In this way, unselfish individuals actually benefit from their altruism.

At least, that’s the theory. While other types of altruism have been observed and modelled (for example downstream reciprocity in which people help others who they have seen acting altruistically), upstream reciprocity has proven harder to pin down, until now

Akio Iwagami and Naoki Masuda at the University of Tokyo have simulated the way upstream reciprocity spreads through a network when the behaviour gradually dies out. It turns out that the type of network is crucial for ensuring the spread of the behaviour. In heterogeneous networks like those that most societies seem to form, upstream reciprocity seems to spread successfully.

But interestingly, Iwagami and Masuda point out that certain individuals seem to benefit more than others. These individuals are “hubs” in these societies, people who have many links to other individuals. That makes sense, of course, because as this behaviour spreads, it is much more likely to pass through hubs than other points on the network.

So the moral of the story is that if you want to benefit from altruistic behaviour, do two things. First, trigger altruistic cascades by performing many acts of unselfish behaviour. Second, become a hub with lots of links to other indviduals.

Ref: arxiv.org/abs/0905.4007: Upstream Reciprocity in Heterogeneous Networks