Boys with the longest ring fingers relative to their index fingers tend to excel in math, according to a new study. In girls, shorter ring fingers predict better verbal skills. The link, according to the researchers, is that testosterone levels in the womb influence both finger length and brain development.
Scientists have been interested for years in the observation that ratios of finger lengths differ in men and women. In men, the ring (fourth) finger is usually longer than the index (second); their so-called 2D:4D ratio is lower than 1. In females, the two fingers are more likely to be the same length. Because of this sex difference, some scientists believe that a low ratio could be a marker for higher prenatal testosterone levels, although it's not clear how the hormone might influence finger development. The 2D:4D ratio has also been fingered in connection with brain-related characteristics--most often in males--such as depression, left-handedness, musical ability, and homosexuality.
In the latest such study, psychologist Mark Brosnan and colleagues at the University of Bath in the United Kingdom photocopied the hands of 74 boys and girls aged 6 and 7. They compared the measurements of the second and fourth fingers with the children's scores on a standard U.K. test of math and literacy. In boys, the lower the ratio, the better their math scores, the team reports in the May issue of the British Journal of Psychology. The boys with the lowest ratios also were the ones whose abilities were most skewed in the direction of math rather than literacy. These differences are small but significant, says Brosnan. With the girls, there was no correlation between finger ratio and numeracy, but those with higher ratios--presumably indicating low testosterone levels--had better scores on verbal abilities.
These sex-specific correlations show how tricky it is to define the roles of sex hormones, says psychologist S. Marc Breedlove of Michigan State University in East Lansing. The range of normal levels of the hormone is different in males and females, so comparable levels would have very different meanings depending on the sex of the individual. And the timing of hormone surges is as important as the levels.
Nonetheless, Brosnan believes finger measurements might be useful not only for gauging the relative contributions of male and female hormones in the womb but also for predicting cognitive abilities--although he acknowledges that "we are not suggesting that finger length measurements could replace SAT tests."
Others are more cautious, pointing out that scientists still have not confirmed that finger ratio is a reliable marker for prenatal testosterone levels. Breedlove calls finger length "a very noisy, imperfect marker at best."