If you really want to know if someone is into you – as in, wants to have your babies – never mind what they say. It’s all in how they say it. A team of researchers led by Juan David Leongómez, a psychologist at the University of Stirling, has discovered that certain “paralingual” features of the human voice, particularly changes to pitch and modulation, can betray the speaker’s hidden sexual attraction to his or her listener. (Or at least, these qualities of the human voice reveal the speaker’s perception of the listener’s attractiveness.) What’s more, these instinctive, unconscious vocal adjustments actually seem to work in terms of increasing the speaker’s odds of getting laid: listeners prefer and respond more favorably (or in technical terms, in a more “proceptive” fashion) to opposite-sex voices that contain these special subtle acoustics than to those that don’t.

In an article soon to be published in the journal Evolution & Human Behavior, Leongómez and his colleagues discovered that when (heterosexual) men, for instance, are asked to flirt with a beautiful woman, two noticeable things begin to happen to their voices. First, their voices get deeper… or rather their voices achieve a deeper minimal octave than under comparison conditions. And second, men’s voices become more sing-songy or pitch-variable when speaking to a pretty woman, sort of like, well, how you’d speak to a baby. It isn’t quite as pronounced as such prosodic “infant-directed speech” (and it’s probably unwise, I hasten to add, for a man to speak to any woman as if she were a puppy), but nonetheless, the investigators found these male voice adjustments during verbal courtship to be an empirically demonstrable effect. What this means is that not only do men’s voices get deeper when they’re chatting up some lovely woman, but they also get higher compared to when their speech is directed at another male or to an unattractive female listener. This effect appeared in both of the language samples tested – native male English and Czech speakers – and even after controlling for the unscripted content of the men’s speech.

So how do Leongómez and his co-authors interpret this puzzling effect? Why, from an evolutionary perspective, of course. “Producing a low pitch at some point during an interaction [with an attractive female] might provide sufficient indication of physical masculinity while freeing men to ‘play’ with their pitch… because low-pitched masculine voices might be associated with aggression, such modulation could potentially enable men to signal both their masculinity and lack of threat simultaneously.” In other words, “Hey, sexy lady, hear this? Hear how I’m sounding right now? That’s right: I’m a virile, testosterone-fuelled male specimen of our species but, cross my heart, I’ll be sweet to you… and our future offspring.” And indeed, a group of naïve female listeners asked to judge the verbal recordings of male wooers found those with the most significant pitch variability the most attractive.

Interestingly, this so-called paralinguistic courtship modulation effect didn’t occur in women’s voices when they believed that they were speaking to a good-looking man, but it did occur when they were speaking to an attractive woman. That’s to say, when (heterosexual) women thought that they were communicating with an especially pretty member of the same sex, they began to stress their pitch modulation. The reason for this isn’t entirely clear, but it could be, as the authors suggest, that these female speakers’ intended audience is in fact desirable male mates, such that women are attempting to enhance their vocal appeal relative to these highly desirable female competitors. “Pfft. She’s not all that,” in other words. “Check out my natural speaking range.” But that itself doesn’t address the adaptation question. Why would such tonal variability in the free speech of human females be attractive to potential male suitors in the first place? On this, the researchers can only speculate. “Such variability might serve as a marker of social interest,” they write, “or help to capture the attention of the listener, or could more simply reflect genuine autonomic arousal in the speaker.” Future work should be able to better tease apart these alternative explanations, but in any event, these new findings by Leongómez and his colleagues add to a growing body of data showing that the remarkably subtle features of our voices are routinely broadcasting covert messages about our hidden reproductive value and interests.