The sexy son hypothesis in evolutionary biology and sexual selection—proposed by Ronald Fisher in 1930—states that a female's ideal mate choice among potential mates is one whose genes will produce male offspring with the best chance of reproductive success and implies that a potential mate's capacity as a parental caregiver or any other direct benefits the father can offer the mother such as nuptial gifts, or good territory are irrelevant to his value as the potential father of the female's offspring. Fisher's principle means that the sex ratio (except in certain eusocial insects) is always 1:1 between males and females, yet what matters most are her "sexy sons"' future breeding successes, more likely if they have a promiscuous father, in creating large numbers of offspring carrying copies of her genes. This sexual selection hypothesis has been researched in species such as the European pied flycatcher.