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After six years of marriage, Ashton Kutcher and Demi Moore are splitting after rumors of infidelity. Here, they are shown at the launch party for

Demi and Ashton (you don’t really need their last names, do you?) have called it quits. Or, rather, Demi has called it quits, apparently, following rumors of Ashton’s moments with a-woman-not-Demi while visiting San Diego.

I write that not because you necessarily care about Ashton and Demi – though a lot of people seem to – but because it brings up the issue of monogamy and if we humans are truly built for it.

The full answer, as you might expect, is pretty complicated. I am currently writing a book with Emory University neuroscientist Larry Young, one of the world’s leading experts in pair bonding -- the way biologists talk about “love” and monogamy in animals -- that attempts to lay it out. But the short answer depends on two things: First, what do we mean when we say monogamy? Second, what’s going on in our brains?

According to Young, only about 3 to 5 percent of mammals form pair bonds between males and females. But even among those that do, “monogamy” does not necessarily mean sexual exclusivity. It means the partners share a social glue, raise a family together and comfort and defend each other. They might very well have sex with the neighbor critter down the block, though.

"Whether humans are monogamous by nature is debatable, and a matter of semantics," Young said.

Monogamy resides in the brain. Young studies voles, small, furry critters found all over North America. One species, prairie voles, is generally monogamous. Another species, the meadow vole, is not. These two species look virtually identical, and even when you look at their genes, there’s barely any difference. But subtle variations in parts of key, brain-related, genes make one monogamous and one promiscuous.

Even within prairie voles, there’s variation. Some are faithful, some play the field. Mounting evidence suggests this is true for people, too.

We know there are differences between human genders, too, with men reporting higher rates of infidelity than women (though women have been slowly catching up). There are several reasons why this might be so, but one is fundamental: Men, especially younger men, have evolved to be readily turned on. Female libido can vary significantly by cycle day.   

We don’t like to think that something we regard as so basic depends on a couple of molecules in our heads, the action of which can be determined by how we develop in our mothers’ wombs, or certain life events, but such forces do act on our brains, making us more or less likely to have extra-monogamy sex.

It’s not, as Demi suggested in her press statement, just a question of “values” or “vows.” How monogamy plays out for each of us also has a lot to do with how we are wired.     

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