What’s the first thing that captures a man’s attention when he looks at a woman? Most commonly, it’s her breasts.

Scientists have verified this by hooking men up to eye-tracking devices and showing them pictures of nude women. When they do this, nearly half of men (47 percent) fixate first on the chest, with the midriff coming in at a somewhat distant second (33 percent).

This fascination that men have with boobs has generated a lot of scientific interest. In fact, researchers have proposed numerous explanations over the years to account for it.

Let’s take a look at the most prominent theories that have emerged to date.

First, some believe that there’s a Freudian explanation, with “mommy issues” being the root cause.

The thought here is that men are essentially acting on repressed sexual desire for the mothers who nourished them at the breast during infancy. In other words, perhaps men see women’s breasts as an opportunity to get closer to their mothers on some level.

Not everyone buys this idea, though. Instead, many think that an evolutionary explanation is far more likely.

And far less creepy.

For example, some scientists have argued that men have evolved to spend a lot of time looking at breasts because they offer information about a woman’s fertility status. The sheer presence of breasts signals sexual maturity, while their size may provide further information about reproductive potential, with larger breasts supposedly signaling better fertility.

An alternative theory—and one that can help to explain why larger breasts are preferred in some cultures—is that men have evolved an attraction to breasts because their size is an indirect signal of a woman’s access to resources. Breasts are primarily composed of fatty tissue, which means that women with larger breasts probably have more reliable access to food, something that would theoretically increase the odds of her own survival and the survival of her children.

If this is true, men’s desire for larger breasts should be most pronounced in environments where access to food or resources is scarce. Consistent with this idea, research has indeed found that men who come from a lower socioeconomic status (SES) prefer larger breasts than men from a higher SES.

Not only that, but research has also found that when men are hungry (compared to full), they prefer larger breasts, too.

Yet another evolutionary perspective, known as genital echo theory, suggests that men like breasts because they evolved to mimic the appearance of the buttocks.

The theory goes something like this: Before humans started walking on two legs, the buttocks would have been the biggest source of men’s visual sexual excitement. However, standing upright necessitated moving the primary sexual cue from the rear to the front of the female body in order to be closer to eye level.

To the extent that this idea is correct (i.e., that the breasts evolved to be large and pendulous in order to serve as a constant sexual cue), it might explain why humans are the only primate species with a permanent, prominent breast. In all other primates, the breasts only protrude from the body when mothers are nursing their young.

One final evolutionary theory worth mentioning (advanced by Dr. Larry Young in his book The Chemistry Between Us) is that perhaps men evolved to love boobs because breast stimulation helps them to bond with female partners.

Of course, breast stimulation is pleasurable for most women (and, in and of itself, can potentially lead to orgasm); however, it also leads to release of the hormone oxytocin, which has been linked to the development of numerous social bonds, including the bond between mother and child, as well as the bond between sexual partners.

Thus, perhaps men evolved an attraction to breasts because it taps into this neurochemical bonding system that promotes feelings of closeness and intimacy.

Although each of these theories is fascinating, keep in mind that they all have limitations. Complicating almost all of them is the fact that not all men love breasts, and what men find attractive about the female body is known to vary a bit across cultures.

In all likelihood, multiple explanations are probably true. In fact, for a very small number of men, I don’t doubt that mommy issues might be a potential contributor— but that’s likely the exception rather than the rule. For the most part, I think the smart money is on the evolutionary side here.

Justin Lehmiller, PhD is a sex educator and researcher at Ball State University and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.