For decades, dating experts have written about the subtle body language cues to look for when trying to determine if a potential mate is into you.
But research released today indicates the major sign of sexual chemistry when people meet for the first time is to do with the voice.
Anthropologists from the UK's University of Sussex trawled speed dating events and recorded potential matches chatting to see if there were patterns they could identify.
Lead author Dr Katarzyna Pisanski said men were found to lower the pitch of their voice when they found someone they wanted to see again in the future — and the tone deepened even further if the lady was particularly desirable.
And while that's interesting, the really fascinating results concerned women while they were interacting with blokes.
"Women spoke in a higher-pitched and less monotone voice on speed dates with men they chose as potential mates," Dr Pisanski said.
However, if they sensed that a man was in strong demand, the higher-pitched vocals switched to significantly deeper pitches.
"Several studies suggest that a lower pitch in women is perceived as attractive or 'sexy'," Dr Pisanski said.
"Women with relatively high-pitched voices are typically perceived as more feminine, younger and more attractive than are women with low-pitched voices. By contrast, women with low-pitched voices are consistently judged as more dominant, competent and mature.
"The trade-off implied by this dichotomy suggests that women may raise their voice pitch to signal youth and femininity, but lower their pitch in contexts where they wish … to indicate sexual interest to a listener."
When there's less competition, a woman's voice tends to be higher-pitched, but when the stakes are high it went deeper, the research suggested.
The human voice has been shaped in a big part by evolution, which can affect how people choose a mate and even compete with rivals to win someone's affections.
However, most of the research to date on this phenomenon has focused almost entirely on men to see how voices shape their sexual selection.
Findings to date include the link between low-pitched voices and levels of testosterone, as well as female attraction to more masculine and dominant-sounding pitches.
"In fact, men with low-pitched voices tend to have higher success in a range of social contexts, from mating to socio-economic and political," Dr Pisanski said.
But how women's voices change involuntarily when they find a potential match hasn't been explored in depth — until now.
The men in the samples were found to prefer those women with deeper voices, suggesting why we change vocal behaviour to attract potential mates.