Sexy son hypothesis – Why females find cheating males attractive

In the 19th century, Charles Darwin and Alfred Russel Wallace were both fascinated by the striking appearance of some animals, particularly males. The theory of natural selection did not seem to be able to explain why male peacocks have such elaborate tails or why stags have large antlers. Something else must be going on.

By the 1870s, Darwin had come up with a working hypothesis: such traits were a consequence of a special type of natural selection called sexual selection. Natural selection is based on the idea that a species evolves to cope with pressures in its environment – for instance, some animals will evolve leaner, more muscular bodies to outpace a natural predator. Sexual selection suggests pressures can come from within the species too. If, for example, females of the species prefer to mate with males that have brighter fur or feathers, then this preference might eventually lead to the evolution of brighter males.

An important point is that the pressure to be sexually desirable can be so strong that males might evolve features that appear to reduce their chances of survival. It seems strange for a male bird to evolve red feathers that are conspicuous to predators, but the pros of sexual attractiveness and mating opportunities might outweigh the cons of being more likely to be eaten.

Ronald Fisher refined the idea in 1930. By this point, the concepts of natural selection and genetic inheritance had come together into a powerful evolutionary model. This meant that biologists had begun to think of organisms and evolution in terms of genes. Fisher realized that females might boost the chances of their genes surviving and multiplying by choosing to mate with attractive males, precisely because any male offspring from these unions may inherit their father’s attractiveness and his likelihood of finding many opportunities to mate. This updated view of sexual selection became known as the sexy son hypothesis.

Fisher’s sexy son hypothesis makes sense of one of life’s enduring mysteries: why females of the species (including the human species) often seem to be attracted to unfaithful males. Females of many species seem to accept that they must devote time to raising their infants. This means they will have relatively few children and their genes won’t spread far. But if male children inherit their father’s attractiveness and his willingness to sleep around, the female should end up having dozens of grandchildren. Ultimately, her genes (many of which are carried by her sexy sons) will multiply.

us males in the expectation that the sons from such unions will inherit their father’s attractiveness and his licentiousness.

These “sexy sons” should spread their genes (which include many of their mother’s genes) far and wide.