15 February 2012

More prenatal testosterone makes ‘masculine’ boys

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The link between prenatal hormone exposure and the facial features we associate with masculinity and dominance in adult men can be observed in early childhood.
Finger length facial shape boys

Credit: Sonja Windhager, Katrin Schaefer

PHILADELPHIA: The link between prenatal hormone exposure and the facial features we associate with masculinity and dominance in adult men can be observed in early childhood.

Scientists have known that ‘masculine’ facial characteristics are linked to high levels of prenatal testosterone, and this can be predicted by finger lengths. But a study published in Proceedings of the Royal Society B today has shown that this is expressed in boys before the onset of puberty, which is earlier than previously thought.

The finding could shed light on social perception of dominance in childhood and the evolutionary basis of a relationship between prenatal environment and face shape.

“We have discovered that in young boys, in parallel to what happens in adult men, exposure to prenatal testosterone predicts shape variation in their faces,” said Katrin Schaefer, professor of anthropology at the University of Vienna in Austria and lead author of the study. “We now know that the prenatal environment influences the shape of the face in male children as well as adult men, so the association is present before puberty.”

Finger length as proxy for hormone exposure

It is well established that early exposure to sex hormones, such as testosterone and oestrogen, exerts an effect on the face shape of adult men and women.

Several studies have revealed that people who were exposed to high levels of prenatal testosterone – as indicated by a low second-to-fourth digit ratio (2D:4D) or the relative length of index to the ring finger – end up with a more ‘masculine’ face. That means they have a wider jaw, thicker eyebrows, a wider and shorter nose and a shorter forehead: characteristics found to be associated with masculinity and dominance among adults.

To test whether or not the link between prenatal environment and robust face shapes is only activated during puberty, the researchers first calculated the 2D:4D ratio on the right hand of 17 Austrian boys aged four to 11 years and took photographs of their faces.

Those images were annotated with landmark points to mark out various morphological features on each face. Finally, an analysis was performed in order to quantify how much of the variation in face shapes was determined by the boys’ 2D:4D ratio.

The team’s results showed that even before the onset of puberty, the lower a boy’s 2D:4D ratio, the more masculine his face shape, which means the effect of the prenatal environment on face shape is expressed earlier than previously thought.

Facial patterns apparent before puberty

The researchers found that the finger length ratio explained 14.5% of variations in face shape. Most of the variation is explained by the boys’ different ages and family backgrounds, but that figure is significant, they said, concluding that the adult male facial characteristics that elicit attributions of dominance are determined in early childhood.

The researchers pointed out that many facial changes result from the secretion of sex hormones during puberty. But despite the effects of adolescence on a person’s face shape, the results show that variation between males due to their prenatal environment is already present before adolescence. “So the effects of circulating hormones in adolescence might be said to be superimposed on the pattern established before birth,” said Schaefer.

Sizing up the sample

Schaefer is now planning to study whether boys with low 2D:4D and more masculine faces are perceived as more dominant by their peers, parents and teachers. “It would be important to know whether this also applies to children,” she said. “If so, they would be treated differently. That would be important for society to know about.”

The research also poses interesting questions about the evolutionary basis of the link between prenatal environment and face shape, she said. “It might be the case in that dominant mothers produce higher levels of testosterone, which results in their offspring having masculine facial characteristics that better prepare them for competition,” said Schaefer. “We don’t know but it would be interesting to find out.”

David Puts, assistant professor of anthropology at Penn State University in the U.S., found the study intriguing, but suggested that the results should be interpreted with some caution because of the small sample size. “In these sorts of studies you can get some statistically significant correlations just due to random chance,” he commented.

“Seventeen participants is quite a small sample, which makes that more likely. Also, the kind of analysis they did to boil down all the facial measurements generally requires a larger sample size. So although I think their methods are good and I wouldn’t discount the results, I’d like to see them replicated in a larger sample.”

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