This month, a team of scientists at Stanford University has reported some of the best evidence yet that testosterone directly influences immune system function in men. … This finding that testosterone may dial down the immune system in humans is consistent with the results of studies of other animals, ranging from fish to chimps. But why would an essential male hormone deliberately handicap the immune system?
The answer might be that this is one of those odd outcomes that follow from the perverse incentives of evolutionary logic. In 1992, a pair of biologists at the University of Tromsø in Norway proposed the “immunocompetence handicap hypothesis,” which essentially says that males will perform dumb, dangerous stunts to impress females. The idea behind the immunocompetence handicap hypothesis is that, in order to prove their genetic fitness to potential mates, males make a trade-off between a robust immune system and a set of elaborate, testosterone-driven secondary sex characteristics, like brightly colored plumage in tropical birds.