The occurrence of altruism and spite - helping or harming others at a cost to oneself - depends on similarity not just between two interacting individuals but also to the rest of their neighbors, according to a new model developed by psychologist DB Krupp and mathematician Peter Taylor of Queen's University.

Individuals who appear very different from most others in a group will evolve to be altruistic towards similar partners, and only slightly spiteful to those who are dissimilar to them but individuals who appear very similar to the rest of a group will evolve to be only slightly altruistic to similar partners but very spiteful to dissimilar individuals, often going to extreme lengths to hurt them.

Taken together, individuals with 'common' and 'rare' appearances may treat each other very differently, and they believe their models could help explain prejudice in humans and other species. 

"Similar individuals are more likely to share copies of each other's genes and dissimilar individuals are less likely to. As a consequence, evolutionary theory predicts that organisms will often discriminate, because helping similar partners and harming dissimilar ones increase the fraction of the discriminating party's genes in future generations," says Krupp. 

The simulation was developed using inclusive fitness theory - a biological framework that considers how an organism's behavior affects its own reproductive success as well as that of its neighbors. 

"We tend to think of individuals as caring only about what another individual looks, smells or sounds like, but our model shows that the appearance of surrounding neighbours matters tremendously, too," says Dr. Krupp. "This work predicts extreme differences in behaviour between what we call 'common' and 'rare' types of individuals - those that are similar or dissimilar to their neighbours."

Upcoming in Proceedings of the Royal Society B: Biological Sciences.