Sexy son hypothesis.

It states that females are attracted to certain traits in males, because these traits will be attractive in her sons either. But isn't this circular?

Like "I'm attracted to that male because he is the son of an [formerly] attractive male". And you can repeat the process infinitely, going through granddad, great-granddad and so on. Doesn't this hypothesis presuppose some sexy traits that are sexy with no cause?

  • $\begingroup$ It's never only one single factor that influences reproduction. "Sexiness" can play a big role in some species, but other factors have a role as well. Also, it is perfectly possible that a factor that has a big role in the present, could have had a very minor role in the past. $\endgroup$
    – alec_djinn
    Mar 7, 2019 at 9:52
  • $\begingroup$ @alec_djinn I believe in good genes hypothesis and that attractive traits signalize an individual who has greater chances of reproduction taken his attraction per se out. But it's not what sexy son hypothesis is about. $\endgroup$
    – rus9384
    Mar 7, 2019 at 10:03
  • $\begingroup$ @rus9384 I think this is clear from the other answers, but "attractive traits signalize an individual who has greater chances of reproduction" could mean traits that mostly improve survival or fecundity (the 'good genes' hypothesis) but it could also mean traits that mostly improve reproduction success (i.e., chance, especially for a male, to successfully mate: 'sexy son'). It's important to remember that fitness is not just about survival, it's about reproduction. $\endgroup$
    – Bryan Krause
    Mar 7, 2019 at 16:20
  • $\begingroup$ @BryanKrause But sexy son hypothesis is then an addendum to the good genes one? $\endgroup$
    – rus9384
    Mar 8, 2019 at 1:14
  • $\begingroup$ @rus9384 I think you could view it as that, or look at it the other way around and consider good genes to be an addendum to sexy sons. It's likely that specific case examples will be better described by the particulars of one or the other but I don't see any way that they are incompatible. $\endgroup$
    – Bryan Krause
    Mar 8, 2019 at 15:44

3 Answers 3


The process is self-reinforcing but the argument is not circular (no tautology implied).

As soon as some male traits are considered more sexy than others, then there is selection for females to like those traits even more, which causes those traits to raise in frequency, which increases the selection for liking these traits. In other words, in this model, the state where there are no sexy traits in males is an unstable equilibrium.

  • $\begingroup$ "As soon as some male traits are considered more sexy than others" And they are in the first place because they signalize (maybe formerly) good genes? But I agree mostly there is a double selection for traits in species with sexual reproduction. Sexual selection is just a layer above non-sexual selection. $\endgroup$
    – rus9384
    Mar 8, 2019 at 1:19
  • $\begingroup$ @rus9384 they don't have to signal good genes they jut need to draw the females attention, A problem bird researches have is it turns out many female birds prefer males with sexy colorful plastic bands around their legs. Something that just happens to draw a females attention is beneficial in and of itself. $\endgroup$
    – John
    Mar 9, 2019 at 19:53
  • $\begingroup$ @John Well, female birds prefer those male birds who are more noticeable, because even before that preference has developed in them more noticeable male birds had more offsprings. Why? Whom does the female bird notice first? That one male she chooses. Of course then those females who had a preference towards such males actually had an advantage: their male offsprings could have more offsprings. I already though about that about a year ago. $\endgroup$
    – rus9384
    Mar 9, 2019 at 23:18
  • $\begingroup$ @rus9384 Could you please to a reference for the claim that female birds like "more noticeable" male birds? I have already heard something along these lines but I would love to see a reference. $\endgroup$
    – Remi.b
    Mar 10, 2019 at 18:20
  • $\begingroup$ @Remi.b I'd wish to provide a reference but this one idea is the one I came up by myself. Sure, I'm not the first. $\endgroup$
    – rus9384
    Mar 15, 2019 at 7:18

Taken by itself I can agree that it might seem circular. However, in practice, all traits act and occur in a pre-existing background, i.e. in organisms with their genetic background and their living environment. All that is needed is a preexisting difference in fitness (for instance connected to the related good-genes hypothesis), that a new gene for female preference of sexy traits can act on. Then the process can become self-reinforcing. The major difference between the sexy sons hypothesis and the good genes hypothesis lies in whether the traits signal increased chance of mating for future sons or if they signal higher quality/viability of offspring (see e.g. Huk & Winkel 2008). My point is that traits that first signaled the latter (quality) can later signal the former (sexiness/mating chance).

  • $\begingroup$ Well, good genes hypothesis is an intuitive one. With one amendment though: those traits which displayed greater fitness in the past (a few tens or hundred generations ago) appear attractive now. But sexy son hypothesis does not add anything to it. Also "higher quality/viability of offspring" contains "increased chance of mating". $\endgroup$
    – rus9384
    Mar 7, 2019 at 15:30
  • $\begingroup$ @rus9384 Yes, that is my point, that past viability effects of offspring is probably often the original trait that might later get caught up in runaway sexual selection due to sexy son. I agree that higher viability of offspring usually leads to increased chance for mating (unless there is a negative tradeoff between viability and "attractiveness"/display traits), but the opposite doesn't have to hold. You can have "increased chance of mating" without "higher quality/viability of offspring". $\endgroup$ Mar 7, 2019 at 15:50

Consider the difference between "vanilla" natural selection and sexual selection.

On paper sexual selection sounds strange; while straightforward selection says that a trait spreads because it makes an individual better at life meaning they'll have more descendants, sexual selection says that a trait spreads because it makes an individual attractive to potential mates, resulting in more descendants sure but it suggests that "being better at life" isn't required. And indeed sexual selection is often used to explain traits that make an individual worse at life, like the peacock's tail.

What's going on is that there is a first-order and a second-order process of causality, that are related but distinct. Say for instance you have a trait that makes your offspring 10% fitter. Awesome, they'll be 10% fitter. Except that this means the offspring of your mate will also be 10% fitter... which means other individuals have an incentive to be your mate, meaning you can get a better mate who will also give fitter offspring, so actually you might end up with offspring that are 50% fitter. The latter process is sexual selection, and while it's dependent on the first process to happen in the scenario I described, the causality, mechanics and results are different. You can have one without the other; for example if a trait makes you 10% fitter but this is completely undetectable to anybody else, they won't be able to select you as a mate on the basis of it so you won't have sexual selection. On the other hand sexual selection can indeed in theory function with something that doesn't improve fitness on the first order.

You can see the sexy son hypothesis as a third-order distinct process. It's not just that you have an incentive to mate with a person because your offspring will be fitter; there is an additional incentive involved in the fact your offspring will also be more attractive. This isn't a tautology because it isn't a foreordained conclusion; for example if you have a "love red hair" gene but nobody else does then you'll pick a redheaded mate on the basis of their sexiness, but your sons won't be sexier for it. On the other hand if everyone except you has the "love red hair" gene, you have an incentive to get a redheaded mate even though you yourself don't find them sexy (or not for the same reason everyone else does at least).

This process certainly allows for sexiness to have no fitness-based cause but it doesn't presuppose it; indeed one would expect for there to have been a fitness-based cause at the beginning of the chain if not at the end.


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