Is there anything to the belief that spouses are most likely to feel the urge to stray after seven years? Actually, according to evolutionary anthropologist Helen Fisher, it happens a lot sooner, and the reasons for this may go back to the dawn of humanity.
"As it turns out, the standard period of human birth spacing was originally four years. We were built to have our children four years apart and I think that this drive to pair up and stay together at least four years evolved millions of years ago so that a man and a woman would be drawn together and stay together, tolerate each other, at least long enough to rear a single child through infancy," said Fisher, author of "Why We Love."
Following the urge to find a new partner after that four-year period, she says, may have been a way that humans added more variation to the gene pool.
So there is an itch -- it's just a four-year itch, according to Fisher.
"People around the world tend to divorce during and around the fourth year of marriage," she said.
20/20 tried to find out where the myth of seven years came from -- why seven years? One possibility is that it was adapted from an old wives' tale about poison ivy -- that if you ever get poison ivy, the itch will return every seven years. But what Fisher seems to suggest is that humans lack the persistence of poison ivy.
So if the seven-year-itch isn't a total myth, it's just a little mistake in math.
Dawn Ricci is an expert who knows something about infidelity. She's a private eye who's been in business tailing cheating spouses for 14 years.
"In a three-month period," Ricci said, "there were 211 women who hired our company to follow their husbands. There were 109 men that we were hired by to follow their wives."
If Ricci's business is typical, it looks as if men are cheating twice as much as women. But Ricci said women are catching up quickly.
Evolutionary anthropologist Helen Fisher says she doesn't believe women are naturally more monogamous than men.
"I think we're going to come to find, as women become more economically powerful and express their sexuality more during the course of this century, that women are just as adulterous as men," she said.
Fisher's evidence begins in the bird world.
"Warbler females seem to have a call or a song that they give out when their husband is out of town in order to attract extra mates," she said.
And in most mammalian species, Fisher said, both the male and the female are totally promiscuous
According to Fisher, it's rooted in the Stone Age. "Millions of years ago, if a woman had an extra lover, she would get extra meat from that male. She could get extra protection from that male," she said.
And she could get extra sex as well.
20/20 spoke with a woman, who asked to be identified only as Jennifer, who was happily married for five years, but was until recently seeing another man. She said no one, including her husband, can provide everything -- especially good sex.
"You need one that their touch makes every hair on your body stand on end and just makes your heart race a million miles an hour and you haven't left the bedroom," she said.
And when she would leave her lover after a night of passionate sex, she said she didn't have an empty feeling or wonder when she would see him again.
"Complete opposite. It was like, 'I just had amazing sex, and he's gone, thank God. I have the bed to myself. ' "