Evolution does not favour selfish people, according to new research. This challenges a previous theory which suggested it was preferable to put yourself first. Instead, it pays to be co-operative, shown in a model of the prisoner's dilemma", a scenario of game theory - the study of strategic decision-making. Published in Nature Communications, the team says their work shows that exhibiting only selfish traits would have made us become extinct. The eminent mathematician John Nash showed that the optimum strategy was not to co-operate in the prisoner's dilemma game. "For many years, people have asked that if he [Nash] is right, then why do we see co-operation in the animal kingdom, in the microbial world and in humans," said lead author Christoph Adami of Michigan State University. The answer, he explained, was that communication was not previously taken into account. Prof Andrew Coleman from Leicester University explains that this new work suggests that co-operation helps a group evolve. "The two prisoners that are interrogated are not allowed to talk to each other. If they did they would make a pact and be free within a month. But if they were not talking to each other, the temptation would be to rat the other out. Being mean can give you an advantage on a short timescale but certainly not in the long run - you would go extinct." Crucially, in an evolutionary environment, knowing your opponent's decision would not be advantageous for long because your opponent would evolve the same recognition mechanism to also know you, Dr Adami explained. This is exactly what his team found, that any advantage from [selfishness] was short-lived.