Christopher: Alright Lucas, five-word review for a book titled Why is Sex Fun? Go. Lucas: Because… evolution is a wingman? Christopher: That is surprisingly close! But according to the author, Jared Diamond, the real story is way more complicated, and frankly, much, much weirder than that. Lucas: I’m intrigued. When you say weird, what are we talking about? I feel like human sexuality is the only kind I know, so it seems pretty standard from my perspective. Christopher: That’s exactly the point. We’re so deep inside our own fishbowl we don’t realize how strange our water is. Today we're diving into Why Is Sex Fun?: The Evolution of Human Sexuality by Jared Diamond. Lucas: Oh, Jared Diamond! This isn't some pop-psychology guru. This is the guy who wrote Guns, Germs, and Steel. A Pulitzer Prize winner. Christopher: Exactly. He's a polymath—a physiologist, an evolutionary biologist, and he spent decades doing fieldwork in New Guinea. So when he tackles a question like this, he's not just giving you one perspective. He's coming at it from anthropology, biology, and deep human history. He argues that our most private, intimate behaviors are evolutionary anomalies. Lucas: Anomalies? That’s a strong word. What makes us so different from, say, every other creature on the planet? Christopher: Well, to answer that, Diamond asks us to look at ourselves from a different perspective. Specifically, from the perspective of a very confused family dog.

Lucas: A dog? Okay, you have my full attention. How does a dog see our love lives? Christopher: Diamond paints this brilliant picture. Imagine you're the family dog. You understand sex. For you, it's simple. A female comes into heat, she gives off a very obvious smell, every male in the neighborhood knows it, and for a few days, it's a public, no-nonsense affair focused entirely on making puppies. Then it's over. Lucas: Right, very efficient. No drama, no dating apps. Christopher: Precisely. But then the dog looks at its humans, John and Barbara. And it's just baffled. First, they have sex any day of the month. The dog knows Barbara isn't ovulating—there are no signals!—but there they go. He thinks, "What a waste of energy!" Lucas: Huh. I’ve never thought of it as a waste of energy, but from a purely biological, gene-passing perspective, he’s got a point. Christopher: It gets weirder for the dog. John and Barbara have sex even when she's pregnant. To the dog, this is just insanity. The job is done! Why are they still doing this? And then, the ultimate puzzle: they do it all behind a closed door. In private. For the dog, and for most of the animal kingdom, sex is a public spectacle. Lucas: And the final straw for this poor, confused canine? Christopher: The final straw is when John's elderly parents come to visit. The dog hears them having sex, too. But John's mother is long past menopause. She can't have children. The dog is just disgusted, thinking, "Those disgusting humans! They're not even trying to make puppies! They're just doing it... for fun?" Lucas: Wow. When you put it like that, our entire romantic life sounds like a bizarre, inefficient, and illogical hobby. I'll never look at my dog the same way again. So what's the animal kingdom's version of 'normal'? Christopher: Think of the Barbary macaque. When a female is fertile, she doesn't hide it. She actively advertises it. And then she proceeds to copulate with every single adult male in her troop, right out in the open. There's no coyness, no privacy. It's a community project. Lucas: A community project! That’s one way to put it. So we have concealed ovulation, sex for fun, sex in private, and sex long after fertility ends. Diamond’s point is that these aren't just quirks; they are fundamental evolutionary puzzles that demand an explanation. Christopher: Yes, because sex is costly. It takes a huge amount of energy. It makes you vulnerable to predators. In the human world, it can lead to fights, jealousy, and social chaos. For natural selection to favor this kind of constant, recreational sexuality, the benefits had to be enormous. Lucas: So what was the benefit? Why did we evolve to be the one species that keeps sex private and recreational? What problem did this solve? Christopher: This is where it gets really interesting. Diamond argues it's a solution to a fundamental conflict that lies at the heart of reproduction. A conflict he describes as an evolutionary "game of chicken."

Lucas: A game of chicken? That sounds... confrontational. I thought reproduction was supposed to be a team sport. Christopher: From an evolutionary standpoint, it's anything but. Think about it. At the moment of fertilization, both the male and female have a choice. Who is going to invest the time, energy, and resources to raise this child? Diamond uses this great metaphor: it’s as if both parents stare at each other and say, "I'm walking away to find another mate. You can raise this kid if you want, but I'm out." The one who "loses" the game of chicken is the one who stays. Lucas: And in most of the mammal world, the female loses that game by default. She's the one who is pregnant, she's the one who lactates. She's already more invested. Christopher: Exactly. Her investment is huge and guaranteed. The male's is... not. He has no certainty the offspring is his, and his best strategy, genetically speaking, might be to find more females to impregnate. This creates a massive conflict of interest. Lucas: That sounds pretty bleak. Are there examples in nature where this conflict gets really dramatic? Christopher: Oh, absolutely. Diamond gives the perfect example with a bird called the Pied Flycatcher. It's a story of pure deception. In the spring, a male flycatcher will set up a territory, find a nest, and woo a female. They mate, she lays her eggs. He's the model husband. Lucas: Aww, a little bird family. How sweet. Christopher: Hold on. As soon as she lays that first egg, he knows she's locked in. She's committed to incubating those eggs. So what does he do? He flies off to a different part of the woods, finds another empty nest, and starts singing his heart out, pretending to be a lonely, available bachelor. Lucas: No! That is cold-blooded! He’s running a scam! Christopher: He is! He lures in a second, unsuspecting female. She becomes his secondary wife. But here's the catch: he can't be in two places at once. So he spends most of his time and energy helping to feed the chicks of his first, primary family. The second female is left to do most of the work herself. Her chicks are often malnourished, and fewer of them survive. Lucas: That is a brutal story. The male wins by having more offspring overall, but the secondary female and her kids pay a huge price. It really is a battle. Does it ever go the other way? Do females ever run the show? Christopher: They do, and it's even more wild. Look at certain shorebirds, like jacanas. The females are bigger, more aggressive, and more brightly colored than the males. In their world, the females fight each other to acquire a harem of smaller males. Lucas: A harem of males? So the roles are completely flipped. Christopher: Completely. A victorious female will lay a clutch of eggs for one of her males, and then she's off. He is now the single dad, responsible for incubating the eggs and raising the chicks all by himself. Meanwhile, she's off laying eggs for the other males in her harem. She basically outsources all the parental care. Lucas: That's incredible. So depending on the species and the circumstances, either the male or the female can end up holding the bag, so to speak. It all comes down to who has more to lose by walking away. Christopher: Precisely. And this brings us back to humans. Our babies are uniquely helpless for an incredibly long time. A human mother in our ancestral environment couldn't possibly raise a child alone. She needed help. She needed the father to stick around. Lucas: So our "weird" sexuality—the constant receptivity, the recreational sex—was the solution? It was the glue to keep the man from flying off to another territory like the Pied Flycatcher? Christopher: That's the leading theory, the "daddy-at-home" theory. By concealing ovulation, a woman keeps the man uncertain about when she's fertile. His best strategy is to stick around and have sex with her frequently to ensure he's the father. Recreational sex then strengthens that pair bond, making them a more effective parenting unit. It turns a potential conflict into a cooperative venture. Lucas: So our capacity for love and bonding isn't just a nice feeling. It's an evolutionary strategy to win the game of chicken by making both parents stay. But if it's a game, how do we signal to a potential partner that we're a good bet? That we have good genes worth investing in? Christopher: Ah, now you're asking the million-dollar question. That leads us to Diamond's most provocative and, for some readers, controversial idea: the evolution of our bodies as a form of "truth in advertising."

Lucas: Truth in advertising? That sounds like something from marketing, not evolutionary biology. What does it mean in this context? Christopher: It means that some physical traits evolve to be honest, reliable signals of an individual's quality—their health, their genes, their fitness. And for a signal to be honest, it has to be costly. Lucas: Costly? What do you mean? Christopher: Think of a peacock's tail. That massive, iridescent tail is incredibly expensive to grow and maintain. It requires a ton of energy, and it makes the peacock much more visible to predators. It's a huge handicap. Lucas: Right, it seems like a terrible idea for survival. Christopher: But that's the point! The peacock is essentially saying, "Look at me. I am so healthy, so strong, and have such superior genes that I can afford to waste all this energy on this ridiculous, beautiful tail and still survive. You should mate with me." The handicap is the advertisement. It's a signal that can't be faked. Lucas: Okay, I get the peacock. So what are the human equivalents? What are our costly signals? Diamond must have some controversial examples. Christopher: He goes straight for the most obvious and talked-about one: the size of the human penis. He points out that, compared to our closest primate relatives like gorillas and orangutans, the human penis is conspicuously large, far exceeding what's needed for the mechanical function of delivering sperm. Lucas: Whoa, hold on. So he's applying the peacock's tail logic to human anatomy? That's a bold claim. Christopher: It is, and it's one of the more speculative parts of the book that critics have pointed to. But the logic follows the handicap principle. Diamond's hypothetical argument is that developing and maintaining this larger organ requires energy that could have gone to, say, building more muscle or a bigger brain. A man with this feature is, in evolutionary terms, boasting. He's saying, "I'm so genetically superior that I can afford to divert resources to this non-essential ornament." Lucas: That is a wild thought. But who is this signal even for? Is it for women, or is it, as Diamond cheekily suggests, more for other men in the locker room? Christopher: He argues it's likely for both. Just like a stag's antlers are used to attract females and intimidate rival males, many sexual ornaments serve a dual purpose. They are about both attraction and competition. It's a signal of virility and status within the male hierarchy. Lucas: This feels like it connects back to the whole idea of men's roles. In another chapter, Diamond challenges the simple view of men as just providers, citing studies of hunter-gatherer societies where the best hunters often share their prize catch widely to gain status and attract other mates, not just to feed their primary family. It’s all about showing off. Christopher: Exactly. It's all part of the same complex tapestry. These signals, whether it's bringing home a huge kill or possessing certain physical traits, are all forms of advertising in the high-stakes market of reproduction. And it applies to women too. Diamond argues that features like permanently enlarged breasts, which are unique among primates, also function as a costly signal of a woman's health and fat reserves—her ability to sustain a pregnancy and nurse a child. Lucas: So our bodies are basically walking, talking billboards, advertising our genetic quality to potential partners and rivals. It’s a bit unsettling to think our attractions are guided by these ancient, subconscious signals. Christopher: It is. But it also explains why we find certain things beautiful or attractive without always knowing why. It's not arbitrary. It's rooted in an ancient language of survival and reproduction, a language of honest signals.

Lucas: So, putting it all together... we have this bizarre, private, recreational sex life because our babies are so helpless. This weirdness is the evolutionary glue that convinces a father to stick around. And our bodies evolved these costly signals to advertise our quality in this high-stakes parenting game. Christopher: That's the core of it. The answer to "Why is sex fun?" is that the pleasure, the constant desire, the emotional bonding—all of it—serves a profound evolutionary purpose. It's the mechanism that helps us navigate the inherent "battle of the sexes" and encourages the immense cooperation needed to raise a human child. Fun is the solution to an evolutionary dilemma. Lucas: It’s fascinating because it reframes everything. Love, attraction, jealousy, even the structure of a family—Diamond forces you to see them not just as cultural or emotional experiences, but as strategies in a biological game that's been playing out for hundreds of thousands of years. Christopher: And what's so powerful about Diamond's work is that he makes you feel the weight of that history. He shows how we are still deeply influenced by these ancient evolutionary pressures, even in our modern world of dating apps and genetic testing. Lucas: So the next time we think about love and relationships, maybe the question isn't just 'is this person right for me?' but 'what evolutionary game are we both playing?' It adds a whole new, and slightly intimidating, layer to everything. Christopher: It really does. It doesn't take the magic away, but it adds a profound layer of understanding. It's a reminder that we are animals, but a very, very peculiar kind of animal. Lucas: We'd love to hear what you all think about these ideas. Is our sexuality a beautiful mystery or a cold, calculated evolutionary strategy? Find us on our social channels and let us know. Christopher: The book is Why Is Sex Fun? by Jared Diamond. It's a short, provocative, and absolutely mind-bending read. Lucas: This is Aibrary, signing off.