Two male researchers have attempted to answer the age-old question: why are men obsessed with women's breasts?

Larry Young, a leading expert in the field of neuroscience of social bonding, and Brain Alexander have dissected the emotional, biological and cultural reasons behind the real reason men love breasts in their new book The Chemistry Between Us: Love, Sex, and the Science of Attraction.

The authors say that one of the reasons why heterosexual men are so fascinated by women's breasts is because of a simple hormone released during breastfeeding that helps form the powerful bond between mother and baby. This hormone is also responsible for creating the evolutionary drive for strong nurturing bonds between lovers, according to the authors.

Dr. Young, who is one of the world's leading social neuroscientists, wrote in a column for the Huffington Post that "biologically speaking" the human male's obsession with breasts is "pretty weird".

"Men are the only male mammals fascinated by breasts in a sexual context," Young wrote. "And women are the only female mammals whose breasts become enlarged at puberty, independent of pregnancy. We are also the only species in which males caress, massage and even orally stimulate the female breasts during foreplay and sex."

Past research has proven that women do in fact enjoy attention on their breasts. Researchers Roy Levin, of the University of Sheffield, and Cindy Meston, of the University of Texas, surveyed 301 participants, which included 153 women, on breast stimulation, and found about 82 percent of women were even more sexually aroused when their breasts or nipples were stimulated and nearly 60 percent ask to have their nipples touched.

The authors cite the success of food-chain Hooters, men's magazines like Playboy and Hustler and about "100,000 years of art" as obvious evidence that men are "extremely" drawn to breasts, and Young and Alexander are sure that the attraction is nature, not nurture.

"Boys don't learn on the playground that breasts are something that they should be interested in. It's biological and deeply engrained in our brain," they wrote.  "In fact, research indicates that when we're confronted with breasts, or even breast-related stimuli, like bras, we'll start making bad decisions (and not just to eat at Hooters)."

Because full breasts are mostly made up of fat, some evolutionary biologists have suggested that big breasts signaled to a man that a woman is in good health and therefore a "top-notch prospect" to bear and raise his children, according to the authors.

The authors also cite another hypothesis based on the idea that many primates have sex with the male entering from behind. Some scientists believe that because male primates copulate with female primates by entering from the back, it could explain why some female monkeys "display elaborate rare-end advertising". These scientists argue that a woman's breasts may have evolved to reflect the contours of her back-side.

However, Young and Alexander reject both of these theories, and instead reference a neurological process that links to same brain mechanisms used to promote a powerful bond between mother and child.

"When a woman gives birth, her newborn will engage in some pretty elaborate manipulations of its mother's breasts," they wrote.  "This stimulation sends signals along nerves and into the brain. There, the signals trigger the release of a neurochemical called oxytocin from the brain's hypothalamus."

"This oxytocin release eventually stimulates smooth muscles in a woman's breasts to eject milk, making it available to her nursing baby," they said. "But oxytocin release has other effects, too. When released at the baby's instigation, the attention of the mother focuses on her baby. The infant becomes the most important thing in the world."

The authors say that oxytocin and dopamine act together to help "imprint" the newborn's face, smell and sounds into the mother's reward circuitry, making breastfeeding and nurturing a pleasurable experience that will motivate her to keep doing those activities to strengthen the mother-infant bond.

"This bond is not only the most beautiful of all social bonds, it can also be the most enduring, lasting a lifetime," they wrote.

They wrote that when a lover touches, massages or nibbles a woman's breasts, it sparks the same process of brain events as nursing.  They explain that humans are also among the very few animals that have sexual intercourse face-to-face, "looking into each other's eyes."

This "quirk" in human sexuality has evolved to "exploit the ancient mother-infant bonding brain circuitry as a way to help form bonds between lovers," they wrote.

The researchers explain that because the release of oxytocin forces the brain's attention to a partner's face, smell and voice, the combination of oxytocin release during breast stimulation, and the increase of dopamine from the pleasure of foreplay and face-to-face sex, helps to forge an "association of the lover's face and eyes with the pleasurable feelings, building a bond in the women's brain".

Young and Alexander conclude that a man's obsession with breasts is far from creepy and is actually "an unconscious evolutionary drive" that helps humans forge loving, nurturing bonds.