2D : 4D Ratio

One extensively studies anatomical feature that is due, in part, to prenatal testosterone involves the ratio of the lengths of the index and ring fingers. Having a large numbers of receptors for testosterone, higher levels of prenatal testosterone, low level of estradiol, or fewer receptors for estradiol – features more characteristic of male features – lengthens the ring fingers by a small amount. Hence, most males have a slightly smaller index finger than ring finger. The lengths of the index and ring fingers of most girls – who have fewer sensitive receptors for testosterone, lower prenatal levels of testosterone, higher levels of estradio, and more receptors for estrogen – are roughly equal. This sex difference is present in human fetuses and other mammals. Gerianne Alexander of Texas A&M University found that the four-month-old infant males described by their mothers as becoming extremely upset when anyone imposed limitations on their actions – about l5 percent of the boys studied – had higher levels of testosterone in their saliva than the majority of boys.

The ratio of length of the index over the ring finger, called the “2D:4D ratio” is under genetic control and has a host of modest correlations with physical and psychological traits. Adult males with masculine ratios of .95 to .97 (the index is slightly shorter than the ring finger) are a little stronger, bolder, aggressive, and more likely to be a member of versity athletic team than males with higher, more feminine ratio. Men with extremely masculine ratios, less than .95, have unusually broad faces, and, if they are portfolio manager, they make more money for the clients than men with less masculine ratios. ~ Page 30