Sunday, November 8, 2009

Finger length associated with cooperative behavior in primates

As the fox behaviors are, and human wide faces may be, related to adrenaline, this study says the finger length and cooperative behavior in primates are both mediated by androgens in the womb. "High levels of androgens, such as testosterone, increase the length of the fourth finger in comparison to the second finger," (that's index compared with ring) so you can look at a primate's hand to tell the approximate level of androgen exposure before birth, and then look at the social behavior, not of individual animals but of species.

Species with high levels of in utero androgens (as shown by digit-length ratios) tend to be sluts (the Science Daily article called  it "competitive and promiscuous," but we know what it means), and other species, such as gibbons and many new world species, have a different finger-length ratio and are more monogamous and less competitive.

The article quote a researcher as saying:
"Humans are unique in that they live in large multi-male, multi-female groups, but maintain strong bonds and show high levels of group cooperation in both males and females. In most other species males are competitive rather than co-operative. Research from finger ratios may help us understand more clearly the development of human sociality and its evolutionary origins."
But they never say what typical human digit-length ratios are compared to gibbons.

One also naturally wonders if it can be applied to differences between humans. There are lots of related studies. This one from 2005 says length of the index finger relative to the ring finger is related to testosterone in the womb and predictive of physically (but not verbally) aggressive behavior in men but not in women.

Kids with longer ring fingers have higher Math SATs than verbal, and those with longer index fingers do better at verbal than math. Does testosterone make them better at words or worse at math?

One study in 2000 said finger length ratios, especially in the right hand, predict sexual orientation, all based on the amount of testosterone exposure in the womb. This is another indication of the biological basis of homosexuality. This isn't a genetic cause but an epigenetic and congenital one. It is caused by the accident of how much of which hormones wash over the fetus (this can explain why one twin is gay and the other not; different parts of the uterus may get different concentrations of hormones), affecting the way the brain gets wired and how long which finger grows.

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