Evolutionary biology tells us this story: everything evolved to make us better at reproducing. Everything has a function — and decoration is no exception. The peacock’s elaborate tail seems useless, but actually it tells us how genetically superior the bird must be if it can survive even with that unwieldy mass of feathers.

Wrong, says Yale University ornithologist Richard Prum. In his new book, The Evolution of Beauty, Prum argues instead that natural selection makes sense in a lot of contexts, but when it comes to desire and attraction, many selections are simply arbitrary. It’s not about what makes the animals fly better or run faster, it’s about what the animal itself subjectively enjoys. It’s what makes the animal happy.

The Verge spoke to Prum about his theory of beauty, attractive birds that have evolved to be worse at flying, and the implications of his theory for humans.

The interview has been lightly edited and condensed for clarity.

You push against the idea that every feature evolved to be adaptive, and instead say that sometimes it’s arbitrary and based on what the animal itself likes. One example you give is of the club-winged manakin, a bird that actually evolved to become cooler but less fit. What does that mean, exactly? And how did it happen?

The club-winged manakin is evolving in a way that makes it worse at flying. The male club-winged manakin does this intricate dance with its wings to attract females. But in order to do that dance, it’s evolved so that it’s wing bones aren’t as efficient as the hollow ones we see in other birds.

We discovered from data that the male and female wing bones are both transformed. They’re all extremely robust and wide and distinct. The male with his weird wings at least gets the ability to sing interesting songs, but the female can never benefit from having these worse wing bones because she doesn’t do the dance. The female who has these weird bones never sings. So how could this happen if all evolution was about making you better and better?

I argue that’s an indication that sexual selection can produce a kind of decadence, in which individuals become worse at their survival even as they’re more pleasing to each other.

How could this happen? Is this the sort of process that leads to extinction?

How could the female make herself worse? This can happen because the cost of her mate choice are deferred to her sons and daughters. So by choosing the male that she likes that makes the cool wing-songs with his nifty wing-feathers, she gets sons that are going to be also attractive, but daughters with wing bones that are less equipped to fly. The trade-off is that her daughters may be worse at survival, but her sons will be better at sexual attraction. So that means that her decadent choices would evolve and continue, even though she’s making her offspring less capable.

Yes, theoretically, that can lead to extinction. This process can be halted, but only halted when there are direct costs to her own survival and fecundity, like if she suddenly doesn’t live as long, or can’t find a mate at all. Then there would be sudden natural selection against preference and that could halt the process.

Can you go into more detail about the difference between adaptive selection, or the idea that every trait can be explained by how it helps you survive, versus the theory of aesthetic selection that you prefer, which says that some things just evolved arbitrarily because animals liked them?

So, there are two theories: the adaptive one says that ornaments like a peacock’s tail and preferences for it evolved because they provide objectively better mating opportunities. The peacock’s useless tail evolved because it tells you that the peacock must be really genetically healthy if it can have that handicap and still stay alive. This suggests that ornaments and beauty tell you about the genetic quality of the organism.

Aesthetic selection says that these preferences co-evolve because of the pleasure they provide. It argues that the animal’s subjective experience — not just external forces — can drive pleasure and can drive the evolution of ornament all by itself. So a peacock can evolve to have a big tail because other peacocks like it, not because it signals that it’s objectively better in some genetic sense. But this isn’t what most of my colleagues in evolutionary biology think.

You argue that animals can evolve traits because it brings them pleasure, not because it’s directly adaptive. But can’t pleasure be adaptive in itself? Sexual pleasure, for instance, makes people want to have sex more, which would probably create more children.

That’s another way of explaining away pleasure. Adaptationism doesn’t explain why, for example, some species require so much stimulation in order to feel enough pleasure. If it was merely about reproduction, you wouldn’t need these elaborate repertoires and mating dances. Why is it that a bird of paradise can sit for three hours at a single male display site and somehow still be trying to decide? Why do they need so much stimulus if pleasure was only a mechanism to get you to choose and procreate?

I think evolutionary biology has a “pleasure problem” going all the way back to the Victorians who were very unsettled to the idea that animals, including people, might be motivated by pleasure. It might be anxiety about the power of passion, and so we’ve been going on a long time ignoring subjective experience.

Some traits that we think of as attractive are biologically useful, right? Aren’t wide hips actually useful for giving birth to children? You write that in the beginning these traits served an evolutionary purpose, but then became “unhinged.” What does that mean?

What happens is that desire for the trait itself becomes its own force, divorced from the original point of the trait. In women, yes, wide-set hips are associated with fertility and the capacity to birth children. That’s the evolutionary origin. But now we find wide hips attractive regardless of whether it’s true that they correlate to being better at giving birth. We like it for its own sake.

Or look at the preference for thinness. Supposedly we’re attracted to thinness because some people think that thinness means health, but there are lots of unhealthy thin people. And if suddenly someone told us that thinness had nothing to do with health, many would probably still be attracted to it. We are often attracted to arbitrary things that don’t tell us much about underlying genetic quality. Just look at the cultural diversity of opinions about things that are supposed to be “universal” like breast size or hip size or waist-hip ratio. Most of that literature is the result of getting undergraduate males to look at computerized women on computer screens and then claim that it’s about something universal about human nature.

Throughout the book, you mention various “genetic indicator” studies that we’ve bought into that have been disproved — like you said that there’s little proof that women with a certain waist-hip ratio are actually more fertile or genetically better. Are there studies in this area that you think are robust?

I think the whole field is poorly supported. I don’t think there are any good examples of honest indicator traits in human sexual women. The problem is that evolutionary psychology as a discipline is filled with people whose intellectual program is merely to propagate the idea that adaptation explains human biology. It’s not dedicated to describing the evolutionary history of people and its actual complexity and as a result, it’s really bad science and a lot of it isn’t even science.

What was your goal in writing the book?

By reframing the biology of sex in terms of the subjective experience of individuals, I want to reframe in some sense the way in which we think about our own sexualities. People today, especially adolescents, are growing up in a culture in which these ideas have become so popular that they see each one of their individual flaws or variations as somehow an honest indicator of their true, objective quality. This is a tragedy because I think it affects how people think about themselves, that other people are actually in some way objectively genetically better than they are. That leads to anorexia, that leads to plastic surgery, it leads to all sorts of unpleasantness.

What I would really love is for people to understand that sexual development is not just the process of becoming a kind of sexual object. It’s the process of self-discovery of your own sexual subjectivity, discovering what it is that you want and like and desire and realizing not only that you have the right and the obligation to discover that for yourself, but that that has been a force in the evolution and the origin of the human species and that in doing that you are being some way ultimately human.