Evolution has, in a sense, made men want to do this.
[The Cleavage Countdown: 8 Facts About Breasts] Recent studies have found that nipple stimulation enhances sexual arousal in the great majority of women, and it activates the same brain areas as vaginal and clitoral stimulation.
When a sexual partner touches, massages or nibbles a woman's breasts, Young said, this triggers the release of oxytocin in the woman's brain, just like what happens when a baby nurses.
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. The main job of breasts, of course, is to feed offspring.
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.
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.
"So, maybe the nature of our sexuality has allowed greater access to the breasts." Young said competing theories of men's breast fixation don't stand up to scrutiny.
For example, the argument that men tend to select full-breasted women because they think these women's breast fat will make them better at nourishing babies falls short when one considers that "sperm is cheap" compared with eggs, and men don't need to be choosy.
Attraction to breasts "is a brain organization effect that occurs in straight males when they go through puberty," Young told Live Science.