

Ask a veteran E-Harmony or Match.com user to list her peeves about online dating, and she’ll surely mention the “cropped-ex shot.” Why do so many people include pictures of themselves with seemingly unattached body parts — a slim bangled arm thrown around their shoulder, or hairy fingers around their waist? Those body parts, assumed to belong to an ex, are maddening in part because of what they don’t reveal: the ex herself (or himself).

That’s because we rate another person’s attractiveness, at least in part, on the attractiveness of his or her partner.

Researchers call this mate-choice copying. As I detail in BLONDES, mate-choice copying happens in many animal species, and there’s mounting evidence that human animals are no exception. If that male has an attractive partner, then there must be something worthwhile about him. It’s a wisdom-of-crowds approach.

The latest mate-copying study, published by researchers at Duke University and the University of California Davis, involves 30 male and 30 female volunteers who all described themselves as straight. One group of volunteers was then asked to rate the attractiveness of men and women pictured alone in photos. Another group was then shown pictures of the same men and women paired together and asked how desirable they would find long-term relationships with members of the opposite sex in the pictures. They were told the people in each photograph had been engaged in a long-term romantic relationship but their relationship ended.

These exes were not cropped out.

No surprise to evolutionary psychologists: Both male and female volunteers rated people in the pictures as more desirable when they were paired next to attractive companions than when they were pictured alone. This was true for both men and women. By using cameras to track eye movements during the experiments, the researchers also saw that when volunteers spent more time looking at a potential mate’s unattractive partner, they were less interested in that mate. Interestingly, even though judges were only asked to rate the person of the opposite-sex, they all spent significant time looking at that person’s partner. Having a homely ex can hurt you.

The exception, the researchers found, is if you’re a hot woman. While women downgraded otherwise hot men if they were paired with a dumpy partner, men gave high ratings to the most attractive women regardless of their partners. As found in other studies, women, generally the choosier and more cautious sex, are more likely to rely on social cues such as whether other women find the target guy attractive.

By no means does this warrant that you leave your ex, in full or in part, in your profile photo. Unless your ex is your only asset.