What leads a female to select a male out of so many? Ever since Richard Dawkins talked about the selfish gene, several evolutionary scientists have tried to explain how females choose their partners in most social species.

One of the proposed theories is the hypothesis of the sexy son, which leaves almost no one indifferent to its more than striking name . Let’s see how it explains the process of reproductive selection and how it relates to evolution.

The Sexy Child Hypothesis

The hypothesis of the sexy son is one of the best known evolutionary theories, especially because of its particular name. Although its origins date back to Richard Fisher in 1930, as it is formulated today we owe it to Patrick J. Weatherhead and Raleigh J. Robertson of Queen University, who clarified their postulates in 1979.

This hypothesis states that the selection of a potential reproductive partner by the female takes into account how attractive the offspring that will arise from breeding with one male or another will be . Thus, if the male children are attractive, they will be chosen by other females when they grow up, which will cause the genes of the first female to be passed on from generation to generation.

Within the theoretical framework of the hypothesis of the sexy child, more importance is given to an indirect factor, such as the aesthetic beauty of the potential partner, rather than more direct aspects such as territorial dominance, physical strength or longevity. The female is more concerned with having many offspring than with having children and grandchildren who are advantaged in terms of strength.

The female chooses, the species evolves

Ever since evolutionary biology was first formulated, the idea has been advocated that one of the fundamental factors in the survival and evolution of a species is how its females select the most desirable males.

In connection with this, Richard Dawkins, in his famous book of The Selfish Gene (1976), already defended the idea that in animal species females choose males with the best genes. The males compete with each other so that many females select them, and the females choose one of them, the best one, to have children almost as desirable as their father .

In relation to the sexy child hypothesis, if females select physically attractive males, then their children will be physically attractive as well. This will cause their children to be chosen by other females as well once they reach adulthood and, in turn, will ensure that they have several grandchildren, making their genetic load survive and reproduce.

Sexual conflict

As we were saying, the females try to choose the best male while the males want to reproduce with as many females as possible. In other words, the females go for quality while the males go for quantity.

As males are not the ones who, in most species, have to give birth to their children, they can fertilize several females in a short period of time, having many children of very variable genetic quality . Some of these children will not reach adulthood, but as there are many of them, there will always be the possibility that several of them will manage to reproduce, and thus pass the male genes to the next generation.

In contrast, females, also in most mammal and social species, can only have one or two children at a time. This is why they choose to look for the male with the best characteristics and, within the hypothesis of the sexy child, the one that is more physically fit. Their reproductive success depends directly on how attractive their male child becomes. If he does not attract other females, the mother’s genetic load will not be passed on to the next generation .

It is for this reason that we can understand that the sexual conflict between females and between males is different. Males compete to ensure that only one of them is able to reproduce or, in the most violent species, to survive and have sex with the females in the group.

Females, on the other hand, have different competition, if any. They may have to fight to ensure that the males notice them, although as most of them tend to go for the maximum amount, it is quite unlikely that they will forget to copulate with any of them.

The theory of good genes

Related to the hypothesis of the sexy child we have the theory of good genes, which is directly related to the fact that females select the best males, but without physical beauty being the only factor taken into account . Since they can often only have one or two children in a short period of time, females prefer that the few children they will manage to have have have good characteristics, which will give them evolutionary advantages.

The theory of good genes, especially explained for the case of polyandry, proposes that females who find better males, sleep with new ones to ensure that the eggs or ova they have are fertilized with the best quality male sperm.

The hypothesis of the sexy son and the theory of good genes are related. As with good genes, the sexy child hypothesis assumes the existence of indirect genetic benefits that are capable of offsetting any directly observable reproductive traits, such as physical strength and parental care, to be successful in reproduction.

The main difference between the theory of good genes and the hypothesis of the sexy child is that in the theory of the sexy child it is assumed that this indirect factor is due to asking how attractive children will be based on sleeping with a certain male. On the other hand, the theory of good genes takes into account reproductive success based on the survival of individuals for having advantageous characteristics for the environment or context in which they are found.

Hypothesis of the sexy son in humans

The sexy child hypothesis has been extrapolated to reproductive success in the human species. Research suggests that during the most fertile period of the menstrual cycle women tend to be more attracted to men with traditionally male traits, especially being physically attractive and with traits attributable to having been exposed to higher levels of testosterone during fetal development.

In contrast, when the menstrual cycle has already subsided and women are less fertile, they tend to be attracted to men with more feminine facial characteristics. It should be said that the studies that have reached these conclusions are quite critical, and that these claims are still being discussed.

Physical attraction and multiorgasmia

According to research conducted by Oakland University, published in the Journal of Personality and Individual Differences , it was found that the more attractive the man she mates with, the more women tend to have orgasms, which may seem obvious at first glance.

Having several orgasms when sleeping with someone attractive can be related to the hypothesis of the sexy child. The woman is much more receptive to an attractive partner , which will make, in case of having children with her, this offspring equally selectable and, in turn, guarantee the survival of the woman’s genes.

It has also been seen that the possibility of a female reaching orgasm depends not only on her subjective judgment of how attractive the partner she is having sex with is, but also how attractive the man is seen by the females in the area.

And why not the sexy daughter hypothesis?

So far, the hypothesis of the sexy son has been made from the male line of reproduction . By this we mean that, based on what this hypothesis postulates, the idea of having sexual relations with the most attractive male is to have children that are also attractive and that these, when they reach adulthood, manage to have sex with a high number of women. However, little has been said about having sexy daughters.

Are the females also the recipients of those genes that would make them desirable as their siblings, the sexed children? Is it important that the females are attractive to the males, if they also copulate with any of them?

According to Dr. Raj Persaud and Dr. Peter Bruggen, and taking into account what we have previously discussed in this article, males, or males in most species, tend to be more concerned with quantity than quality , being much less selective in choosing a mate than in comparison to females.

That is why the influence of the female genetic load seems to be less decisive for the males and therefore it does not matter how attractive the female is to them.

Bibliographic references:

  • Sela, Y. & Weekes-Shackelford, V. & Shackelford, T. & Pham, M. (2015). Female copulatory orgasm and male partner’s attractiveness to his partner and other women. Personality and Individual Differences. 79. 10.1016/j.paid.2015.02.008.
  • Huk, T., Winkel, W. (2008), Testing the sexy are hypothesis-a research framework for empirical approaches, Behavioral Ecology, 19,(2) 456-461, https://doi.org/10.1093/beheco/arm150
  • Weatherhead P. J., Robertson R. J. (1979). “Offspring quality and the polygyny threshold: ‘the sexy are hypothesis’. The American Naturalist. 113 (2): 201-208. doi:10.1086/283379. JSTOR 2460199.