Why do straight men devote so much headspace to those big, bulbous bags of fat drooping from women’s chests? Scientists have never satisfactorily explained men’s curious breast fixation, but theorists are gonna theorize. So let’s take a tour of the sexy speculation surrounding the human bosom — with a few stops to explain why it’s so hard to figure out just why breasts hold such allure.

Mammary glands are a defining feature of mammals, but humans seem unique in granting mammaries a large sexual role. That’s not to say interest in nipples is entirely unheard of elsewhere in the animal kingdom: In the book “Biological Exuberance: Animal Homosexuality and Natural Diversity” (Stonewall Inn Editions, 1999), Canadian biologist Bruce Bagemihl notes that a couple of primate species, including humanity’s close relative the bonobo, have been seen stimulating their own nipples while masturbating. Still, few mammals other than humans mate face-to-face (the behavior makes headlines when seen in the wild), so nipple stimulation isn’t generally part of the script.

Researchers have long speculated that humans evolved the fatty deposits around the female mammary glands for sexual reasons. Anthropologist Owen Lovejoy argued that evolution put a bull’s-eye around both female and male reproductive organs in order to promote pair bonding. In this hypothesis, it wasn’t just the female breast that got a lift; men acquired relatively large penises for their body size, too. [Why Do Women Have Breasts?]

Another long-standing theory holds that breasts evolved as a way to signal to men that the woman attached to them was nutritionally advantaged and youthful — and thus, a promising mate. Studies finding that men prefer large breasts and a high waist-to-hip ratio bolster the notion that an hourglass shape communicates youth and fertility. A 2004 study in the Proceedings of the Royal Society B even found that women with large breasts have higher levels of the hormone estradiol mid-cycle, which could increase fertility.

Nature or nurture?

But there are pitfalls to this line of work. For one thing, it’s not actually clear that breasts are universally adored. In a 1951 study of 191 cultures, anthropologist Clellan Ford and ethologist Frank Beach reported that breasts were considered sexually important to men in 13 of those cultures. Of those, nine cultures preferred large breasts. Two — the Azande and Ganda of Africa — found long, pendulous breasts most attractive. Another two — the Maasai of Africa and Manus of the South Pacific — liked breasts that were upright and “hemispherical,” but not necessarily large. Thirteen cultures also reported breast simulation during sex, but only three of those overlapped with the societies where men reported finding breasts important for sexual attraction.

In a chapter in the book “Breastfeeding: Biocultural Perspectives” (Aldine de Gruyter, 1995), cultural anthropologist Katherine Dettwyler describes telling friends in Mali about sexual foreplay involving breasts and getting responses ranging from “bemused to horrified.”

“In any case, they regarded it as unnatural, perverted behavior, and found it difficult to believe that men would become sexually aroused by women’s breasts, or that women would find such activities pleasurable,” Dettwyler wrote.

In the cultural view, men aren’t so much biologically drawn to breasts as trained from an early age to find them erotic.

“Obviously, humans can learn to view breasts as sexually attractive. We can learn to prefer long, pendulous breasts, or upright, hemispherical breasts. We can learn to prefer large breasts,” Dettwyler wrote. [The 7 Biggest Mysteries of the Human Body]

Even if there is some biological underpinning for an interest in bosoms, it might vary by culture. A 2011 study compared men’s preferences for breast size, symmetry, and areola size and color in Papua New Guina, Samoa and New Zealand and found that men from Papua New Guinea preferred larger breasts than men from the other two islands. Because the men surveyed from Papua New Guinea hailed from more of a subsistence culture than the men in Samoa or New Zealand, the results support the idea that in places of scarcity, padded bustlines could signal a well-fed woman with reserves for pregnancy and childrearing, the researchers wrote. Areola size and color preferences were highly idiosyncratic between cultures.

Sexual sideshow?