Since 1990, the number of women who cheat on their husbands has increased by 40 percent, while the number of men has remained relatively the same, according to a new book from psychotherapist Esther Perel, "State of Affairs: Rethinking Infidelity."

Beyond this surprising statistic, women’s motives for cheating are often far from what’s expected, Perel reveals. In her recent Atlantic feature, "Why Happy People Cheat," she explains that a happy relationship is no guarantee against infidelity.

Perel’s work focuses particularly on marriages, although those same findings apply to unwed couples. She’s studied the remarkably different ways that cheating is defined, experienced, and talked about, depending on the time and place the infidelity occurred.

In the U.S., cheating is seen as something catastrophic. We understand the betrayed partner suffers immensely. “It is a shock that makes us question our past, our future, and even our very identity,” Perel says.

And for the first time in our cultural history, this trauma isn’t primarily occurring among women. At increasing rates, ladies are inflicting the damage to their committed partners, and the way they do this is notably unique.

Studies show that when men cheat, they’ll often do so opportunistically — they’ll cheat more often, and with someone less attractive or desirable than their current companion. Ladies in relationships, however, tend to cheat more selectively. When women are unfaithful, they’ll rarely fool around with fellas who are “socially subordinate” to their current partners. Essentially, women are more likely to cheat up, while men are more likely to cheat around.

Beyond gender, there are countless other qualities that determine someone’s likelihood of infidelity. Our age, our job, our sex drive, our impulsivity, and our unique relationship circumstances play a role in predicting loyalty.

More and more, the issue seems to be that marriage and long-term monogamy aren’t meeting our contradictory ideals, Perel points out. “We want our chosen one to offer stability, safety, predictability, and dependabilit," she says. "And we want that very same person to supply awe, mystery, adventure, and risk. We expect comfort and edge, familiarity and novelty, continuity and surprise. We have conjured up a new Olympus, where love will remain unconditional, intimacy enthralling, and sex oh so exciting, with one person, for the long haul."

When these unrealistic expectations fail us, cheating seems like a reasonable excuse to achieve everything we were promised. Or even when our partnership meets every expectation, cheating can be our form of self-discovery — of searching for a new identity. “We are not looking for another lover so much as another version of ourselves,” Perel explains.

Reinventing ourselves is often not a good enough excuse, however. When partners discover betrayal, it doesn’t matter if they’re told the infidelity had nothing to do with them. The damage is still devastating. Then, the question becomes, how to rebuild the relationship?

Sometimes, the resulting mistrust is too much for the relationship to overcome. Sometimes, however, the relationship emerges stronger as a result.