What regulates the lengths of human fingers?

FROM financial traders’ propensity to make risky decisions to badly behaved schoolboys’ claims to be suffering from attention deficit hyperactivity disorder, testosterone makes a perfect scapegoat. In both of these cases, and others, many researchers reckon that the underlying cause is exposure to too much of that male hormone in the womb. Positive effects are claimed, too. Top-flight female football players and successful male musicians may also have fetal testosterone exposure to thank for their lot in life.

Yet the evidence that it is exposure in utero to testosterone that causes all these things relies on a shaky chain of causation. What these people actually share is a tendency for their ring fingers to be longer than their index fingers. This peculiarity of anatomy is often ascribed to fetal testosterone exposure because it is common in men and much rarer in women, and because there seems to be a correlation between the point in gestation when it appears and surges of testosterone in the womb. But the link has never been proved decisively. It has, rather, just become accepted wisdom. …