Men instinctively adopt a sing-song tone when talking to a woman they find attractive, a new study has found.
The study led by the University of Stirling in UK which looked at the vocal ranges of men and women in courtship scenarios found humans make subtle changes in their voices when they speak to people they find attractive.
The research – led by Stirling's Juan David Leongomez - also found that these subtle voice modulations also make the speaker seem more attractive to the person at whom the speaking is directed.
The research team studied a sample of 110 heterosexual individuals, who were either native English or Czech speakers.
They compared the vocal patterns of men and women – when the speakers were talking to people they perceived as attractive, versus people they perceived as unattractive.
When men of both languages were talking to women they found attractive, the researchers found the men tended to speak in a more variable voice (more "sing-songy").
The men also reached a lower minimum voice pitch, or 'deep voice', compared with when they spoke to women they considered less attractive.
The study also shows that people modulate their voices to signal romantic interest and that this, in turn, seems to make the speaker seem more attractive.
"For men, it is important to sound masculine, which is manifested in a deeper voice pitch. However, extreme masculinity is associated with negative traits in a partner, like a tendency for increased aggressiveness and promiscuity," Leongomez said.
"This puts men in a dilemma, because they have to convey two seemingly contradictory messages at the same time: 'I am a masculine man', and 'I'd be a good partner and father.
"The solution may be to vary their pitch - which would explain the sing-songy quality of the voices we observed in men speaking to attractive women," Leongomez said.
The researchers' findings also showed that bystanders respond to these subtle differences too.
When the voice recording of a man speaking to an attractive woman was played to female listeners, the listeners found the voice more attractive than a recording of the same man speaking to a less attractive woman.
"If a woman perceives a man's voice to be more sing-songy, then it is likely the man finds her attractive. However, these vocal modulations are very subtle and probably not produced consciously by the speaker.
"Additionally, whilst they help make a person sound more attractive to the opposite sex, people do not seem to be