Gordon Gallup, an evolutionary psychologist at the University of Albany, has a suggestion for women going on first dates: Bring a handgrip dynamometer. It’s a small, purse-size device they can use to measure their date’s grip strength. Why? Gallup’s new research suggests that handgrip strength in men is directly connected with reproductive fitness.

People with high grip-strength scores are usually healthier than those with weak grips. “They live longer and recover faster from injury,” Gallup says. “They have reduced disability, higher bone density and greater fat-free body mass.” And in a study published this year, Gallup and his son Andrew  at the time an undergraduate psychology major  found that males with high grip-strength scores reported being more aggressive and dominant and had more masculine body types (broader shoulders, narrower hips). They also had “increased sexual opportunities,” which resulted in an increased number of sexual partners, and younger ages of first sexual encounter. (For women, handgrip may be more about sexual protection than prowess: Gallup has found that women’s hand strength increases when they’re most fertile, a trait he says may have evolved to prevent forced impregnation by unwanted mates.)

A logical response to the news that handgrip relates to sexual fitness would be to buy hand-exercising equipment. But that won’t get you far. Grip strength, Gallup says, is largely genetic  certain genes increase testosterone levels, which increase muscle mass. Evolutionarily speaking, its genetic basis may relate to the fact that our primate ancestors traveled by swinging through trees. “Clearly, grip strength is important under those conditions,” Gallup says. “I mean, if you lose your grip on one of those branches, you’re at risk of falling. And falling out of the canopy does not bode well for your future genetic stability.”

In general, Gallup says, what people find attractive are traits that signal the ability to pass along genes to future generations. So female primates evolved an attraction for mates less likely to fall from trees. It’s also possible that males with strong hands were better hunters and protectors.