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.
