When I was in graduate school, a classmate of mine confided in me that she was attracted to a mutual friend’s husband. She didn’t plan to pursue him, but she told me that she found him attractive because he had all the qualities she was looking for in a boyfriend.

He was good-looking, stable, he had a good job and a nice home, and she fantasized about what a relationship with him might be like. Why do women find themselves attracted to men who are already in relationships?

Psychologists refer to this phenomenon as “mate-choice copying,” an effect that has been shown in other animals, including fish and birds. In other species, females prefer males who have been seen with other females or who have mated with other females (Uller & Johansson, 2002). In humans, women show this propensity to copy others’ mate choices more than men (Yorzinski & Platt, 2010).

The idea is that females need to choose a good partner, and the interest of other women can signal that a man has the right qualities—he is, in a sense, “pre-screened.” But women do not always prefer mated men. Below are the conditions under which women find men more attractive when those men are already in romantic relationships.

The Female Partner Must Be Attractive

Interestingly, the attractiveness of the female partner matters more than men's own looks. Regardless of the physical attractiveness of men, women rate men as more appealing when they are portrayed as dating an attractive female partner (Waynforth, 2007; Yorzinski & Platt, 2010). However, this same research shows that the reverse is also true: Men are perceived as less appealing when they are portrayed as dating an unattractive female partner.

Men Must Be Perceived as Potentially Available

Although a good-looking partner may enhance men's attractiveness, men must also be perceived as available in order to truly interest women. Bressan and Stranieri (2008) found that the preference for mated men was strongest when the men were described as having a girlfriend, a bit weaker when they were described as being in love, and the weakest when they were described as being married.

Similarly, Uller and Johansson (2002) reported that women found men more attractive when they were not wearing a wedding ring. Attempts to lure men away from their partners are perceived as less likely to be effective when men are in committed relationships (Schmitt & Buss, 2001).

Some Women Are Not More Attracted to Mated Men

Mate choice copying is less common among women with more sex partners (Waynforth, 2007). Waynforth suggests that women with sexual experience don’t feel like they need to copy others’ mate choices and are confident in their own choices.

Furthermore, Bressan and Stranieri (2008) found that women who already had partners of their own preferred mated men, but only when they were not fertile. When they were in the fertile portion of their menstrual cycles, these women preferred single men.

Although Mated Men are Attractive, Women May Choose Not to Pursue Them

Just as my friend admired our married friend from afar, women may choose to admire these men but not to pursue them as relationship partners. Women who are agreeable and conscientious are less likely to pursue mated men (Schmitt & Buss, 2001). However, women may choose to try to attract mated men when they are particularly dissatisfied with their current relationships and if men display resources (Schmitt & Buss, 2001).