Boys', post-natally, have large concentrations of testosterone, then it rapidly decreases to the same level as girls. Particular effects have now been noted on the digits and robustness of face in 4-11 year-old boys. The relationship between the index (2D) and ring (4D )fingers is controlled by two special alleles (named Homeobox). Facial shape, especially the contrast between small and prominent jaws seems influenced by this "digit difference," and is thought to be influenced by the high hormone level.

This research in the University of Vienna sought to prove an early influence on this face shape. Seventeen boys were photographed and their fingers measured to ascertain the 2D:4D ratio. Konstanze Meindl and his Austrian collaborators then got down to complex analyses. The full research paper has been published today in The Royal Society Journal, Proceedings of the Royal Society B: Biological Sciences.

Results produced digit ratios between 0.91 and 1.01, and the association with face shape explained 14.5% of the variations. The high and low digit ratios are shown in the visualisation above (top image).

A larger forehead and long, slim eyebrows over larger eyes characterises the 1.01 ratio or similar high values. The 0.91 ratio would tend to show up with smaller, shorter foreheads and thicker eyebrows that are further from the smaller eyes. Between the eyes is a larger gap, the flared nostril is shorter and wider and most prominent, the extended lower jaw is prominent.

Jaw-dropping sprinting from these three! Credit: Shutterstock

So this clever research gives a clear indication of one association. The indication is well known for adult men and even the radiographs of a Denver Growth Study show that adult cranial shape might be established in a child's first year or two. The interest beyond this intriguing study extends to low ratio boys sprinting faster and baby faced boys being treated in alternative ways by society. I bet my bottom dollar you are measuring your children already. Who knows what will be associated with 2D:4D next?