What is the sexy son hypothesis in animal mate choice?

The sexy son hypothesis is an evolutionary explanation for female mate choice in which a female selects a mate based on an apparently arbitrary, attractive trait—such as elaborate plumage or a complex song—rather than a trait that directly enhances offspring survival.1

By choosing a male with these attractive characteristics, the female produces sons who inherit the same trait. Because the trait is preferred by other females, these "sexy sons" are more likely to secure mates themselves, thereby increasing the female's reproductive success by producing more grandchildren.1

This process can lead to runaway selection, a phenomenon first proposed by statistician R.A. Fisher in the 1930s, where a preference for exaggerated traits becomes self-reinforcing. While these traits may be arbitrary, they can sometimes overlap with the good genes hypothesis, where the preferred trait also serves as an honest indicator of a male's overall genetic quality.1