We're made to believe marriage is a breeze. Flickr / Simon According to Farnoosh Torabi, author of "When She Makes More: 10 Rules for Breadwinning Women," both the man and the woman in a heterosexual relationship are more likely to cheat when the woman earns more.

But why?

Peter Pearson, relationship psychologist and cofounder of The Couples Institute in Menlo Park, California says, "Most of the time when people cheat it's because they want to feel more alive, they want to feel like they're more attractive; it's an ego thing. It's like waking up from sleepwalking in your marriage."

He says what leads to this "sleepwalking" is "differences and disillusionment."

And a wife outearning her husband falls into that category.

According to Pearson, popular culture teaches us from a very young age that marriage should be "easy, effortless, and enjoyable" if you find the right person.

"Cinderella, she marries the prince, and as far as I can tell, Cinderella and the Prince never had one substantial discussion about anything," Pearson says. "Yet they get married and live happily ever after."

So people come to believe that marriage should be a breeze.

When couples run into differences — such as a role reversal where the wife is the breadwinner in the relationship — they face disappointment and disillusionment, which can then lead to cheating, Pearson says.

He adds that this is the point at which couples come to him and say "I love my partner, but I'm not in love with my partner."

Pearson also says that women who earn more often expect their husbands to pick up slack where household chores are concerned. The problem is, women are reluctant to bring this up to their husbands because they feel guilty about the fact that they earn more. This leads to a lack of communication and seeps into other parts of the marriage.

"If the guy tends to be passive and he doesn't do more in terms of roles and responsibilities around the house, that becomes an even bigger problem because she starts to lose respect for him; her sexual interest starts to decline," Pearson warns.