By Willow Lawson, published on May 1, 2004 - last reviewed on June 9, 2016
Wedding announcements in newspapers rarely tell the whole story. If they did, they would occasionally read something like this: "The happy couple met through her boyfriend at the time, who is the groom's former best friend. It took four months to lure her away."
According to one study, up to 20 percent of long-term relationships begin when one or both partners are involved with others. Evolutionary psychologists call this "mate poaching." This figure holds steady across age groups and among couples who are married, living together or dating, according to psychologists who polled some 16,000 individuals in 53 countries as part of the International Sexuality Description Project. Most surprising to researchers: Sweetheart-stealing is prevalent across continents and cultures, although it is notably less common in East Asia.
In North America, 62 percent of men and 40 percent of women say they've attempted to entice another's mate for a short-term fling. Some 47 percent of men and 32 percent of women say they've succumbed to such attempts. The more sexual equality in a culture, the closer women come to matching men in the number of poaching attempts.
That males seek multiple partners is standard Darwinian theory, borne out in studies across the animal kingdom. But researchers say it is becoming increasingly clear that in humans, women are equally hardwired for infidelity. David Schmitt, lead researcher and psychology professor at Bradley University in Peoria, Illinois, says studies show women, just like men, are designed to be unfaithful and promiscuous at times. "It's usually around ovulation," says Schmitt.
Schmitt has found that certain personality traits are more common among poachers and poachees, regardless of whether they are Estonian, Brazilian or Moroccan. The most prolific mate thieves—both male and female—describe themselves as open to new experiences, sexually attractive and willing to talk about sex. Men and women who have received the most poaching attempts also tend to have these traits. One of the key ways that poaching seems to happen," says Schmitt, "is that you get two people together who are open to talking about their sexual feelings. It's a slippery slope."
Although many people leave onetime partners for a love that lasts a lifetime, it's not surprising that unions that result from sexual double-crossing tend to be unstable. Arlene Goldberg, a Philadelphia psychologist and couples therapist, says the odds are always high that a cheater will cheat again. However, she says, it's worth noting that there are dozens of reasons why people are unfaithful. "The woman who is taking care of her husband with Alzheimer's and is seeing someone on the side isn't having the same kind of affair as the man whose wife just had a baby and who isn't getting as much attention as he used to."