Recent research tells us that the sound of our voices betrays clues about our age, whether we’re menstruating (if we’re female), our sexual behavior and our physical strength. All of these factors, evolutionary psychologists say, are indicators of her reproductive potential. Voice pitch, in particular, has been associated with indirect measures of reproductive fitness in both men and women—men with “masculine” low-pitched voices and women with “feminine” high-pitched voices tend to be rated more attractive and have more sexual partners, for example, and they have higher levels of sex hormones (testosterone in men and estrogen in women).

But there’s been very little research so far on whether the way we speak actually relates to higher fertility, in part because most of this work is done on college students who haven’t started having children yet. In 2007, researchers finally left campus and travelled to Tanzania to study how voice pitch relates to fertility. The Harvard-lead team found that among the indigenous Hadza people, men with low-pitched voices reported having more offspring. This study was a step in the right direction, but it was limited by the fact that without a paternity test, men can’t be sure that the children they’re raising are their biological offspring. In fact, depending on where in the world you look, between 2 and 30 percent of men are—unbeknownst to them—raising children that they didn’t sire.