By Emma Young

Many animals, including sea lions and dogs, can accurately predict the size and strength of a potential adversary in part by listening to their vocalisations – such as the ferocity and depth of their barks or growls. People weren’t thought to be much good at doing something similar. But in previous studies, volunteers were asked to judge the absolute height and strength of another person, based on the sound of an aggressively-spoken sentence or a ‘roar’. Now in a new study, published in iScience, when participants were instead instructed to listen to recordings and judge how much stronger, or weaker, taller or shorter the vocaliser was, compared with themselves, they could do this with a high degree of accuracy.

As the researchers, led by David Reby at the University of Sussex, point out, this is potentially far more practically useful than being able to discern someone else’s absolute height or strength.

The researchers recorded 30 male and 31 female drama or acting students at the University of Sussex, who were instructed to imagine themselves in a battle or war scenario, about to charge and attack, and to utter “That’s enough, I’m coming for you!”, followed by a “non-verbal vocalisation” expressing the same motivation (these usually sounded like roars). The researchers then measured each student’s height and also their flexed bicep circumference and handgrip strength, which were used to produce a single strength score for each person.

One group of listeners rated relative strength. These were 45 men and women recruited from Tromso (and surrounding rural towns) in Norway (all were fluent English speakers), and from the University of Sussex. They also had their own handgrip strength and flexed bicep circumference measured.

A separate group of 56 people from the US rated the vocalisers’ relative height, and self-reported their own heights.

These listeners rated all the vocalisers on a 101 point scale, from -50 (much weaker / shorter) to +50 (much stronger / taller), with the slider’s default position set to 0 (“same as you”).

The researchers found that overall both men and women were fairly good at identifying whether the vocalisers were relatively stronger/weaker and taller/shorter than themselves. For example, when judging roars – which, for male (but not female) vocalisers increased perceived strength more than the aggressive sentence – male listeners correctly rated vocalisers who were substantially stronger than they were as being stronger 88 per cent of the time (and never ranked such vocalisers as being weaker).

But analysis of the finely-scaled judgements made using the 101-point sliding scale also revealed some biases. There was a general tendency for women to over-estimate male vocalisers’ relative strength, and men to under-estimate women’s relative strength.

The researchers say this fits with previous findings of a general tendency for women to underestimate, and men to over-estimate their abilities. However, given that men are, on average, stronger than women, from an evolutionary perspective, it may be safer for a woman to err on the side of caution, and assume that the (invisible) owner of a male voice is significantly stronger, as she’ll be right more often than not.

Overall, though, as has been demonstrated with other mammals, “both male and female listeners can use available formidability cues conveyed in aggressive speech and roars to make ecologically relevant judgements about speakers with a high degree of accuracy,” the researchers conclude.

—Human Listeners Can Accurately Judge Strength and Height Relative to Self from Aggressive Roars and Speech

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest