Whatever the case, the question of trust is front and center. Consider this thread on The OP (original poster) writes that she busted her husband sending sexual e-mails to women he met online, using a secret account. Another woman becomes her been-there, felt-that sounding board. The dialogue has the intimacy of two lifelong friends, not anonymous strangers who just met.

OP: I want to be angry, but to be honest, my heart is just broken.

Reply: Well, don't let it be broken just yet. You need more info. How is the rest of the marriage?

OP: Great. I never suspected it and was in total shock when I saw it. It's completely out of his character. We tried to talk it out. He has no reason, no excuse. It wasn't like he would ever actually see those girls—he says. He says he's genuinely sorry, and I have the sneaking suspicion he wants it left at that. How can I get answers from him? I know there has to be some even if they aren't the ones I want them to be.

Reply: Of course he wants it left like that—to just forget it ever happened. But don't let this be swept under the rug.

OP: I have blocked all sites like the one that got him in trouble. I hate to feel as though I'm just controlling his urge. Shouldn't it not be there at all?

Reply: I wouldn't block the sites; I would watch what he does. You want to find out if he would like to lead a secret life, and you can't just ask him, because they lie.

The psychologist Erik Erikson, who advanced our understanding of human psychosocial development, theorized that because we are born helpless, our first object of trust is our mothers—and that if they live up to our trust, meeting our earliest needs (for food, shelter, affection, attention), we can grow up to have faith in others. (Ironically, Erikson's mother became pregnant with him during an extramarital affair.)

Trust is essential to human experience. A million years ago, while cavemen were out hunting mastodons, their female counterparts were slaving away back in the cave, perpetuating the species and creating clans. Clans became increasingly generous and trusting, leading to the creation of entire cultures.

Paul J. Zak, a professor at Claremont Graduate University in Riverside, California, who specializes in the neuroscience of trust, argues that women spearheaded this evolution. The reason: oxytocin, the hormone that promotes generosity and sociability. Childbirth, breastfeeding, and ovulation cause oxytocin to surge. Men, on the other hand, tend to be heavy on testosterone, which actually blocks the release of oxytocin. In one experiment, Zak found that when researchers boosted oxytocin levels in men's brains via a nasal spray, they became far more empathetic, generous, and eager to entrust strangers with their money. "Oxytocin makes us care for people around us," says Zak, adding that the positive emotions triggered by this feeling of goodwill release even more oxytocin, which he calls the trust hormone.