Marilyn Monroe and Barry White had it sussed decades ahead of the science. Marilyn's breathy vocals takes many men's breath away. And Barry's deep crooning has left millions of women swooning. But why is this?
A paper published in Plos One this week by Yi Xu and colleagues at University College London revealed how you sound sends out signals about your body shape. Marilyn projects petite, while Barry sounds big and strong.
The vocal tract, or airspace above your larynx, shapes the resonance of the sounds produced by your vocal folds and is shaped differently in men and women, a trait thought to be under the control of sex hormones oestrogen and testosterone. These two hormones are also involved in body shape. So the sounds you make might also be linked to what your body looks like.
To test this, researchers took 16 males, 16 females averaging mid 20s in age, asked them to listen to the sentence "I owe you a yoyo," over 100 times.
The sentence was tweaked each time and volunteers had to rate the attractiveness of the sound.
Males rated females as more attractive if they had increased breathiness and dynamics of vowels (called wide formant dispersion). If the voices were made lower, they were less attractive. But increasing pitch didn't increase attractiveness. So a natural pitch seemed best.
Females, on the other hand, preferred a male voice that signalled a large body size via low pitch and monotone vowels. Interestingly women also seemed to prefer a breathy man though, which may soften the aggressiveness associated with large body size.
The evolutionary biology behind this? The vocal folds in our larynx knock repeatedly against one another as air rushes past from our lungs, causing vibrations and producing the voice. The frequency of the sound they produce is affected by their length, mass and tension. In men, the folds are longer and so produce sound of lower frequencies, which we hear as lower pitch; shorter folds in women will have a higher pitch. Females also have more breathiness which comes from air whistling through a gap at the back of the vocal cords. The vocal tract is also longer in men and it's this that makes their vowels more likely to sound monotone.
The scientist's hypothesise that there may also be an evolutionary pressure coming into play with vocal tract shape. Males across the bird and mammal kingdom use low monotone vocal sounds to project a threatening big body size to help successfully warn off predators and compete for a mate.