Evolution, aggression, and men: Hormones matter?

Bryant (mycol1@unm.edu)
6 Jul 1995 20:56:14 -0600

Recently, in a thread on evolutionary psychology, some of us have argued
a while about whether the universal sexual dimorphism in human aggression
is the legacy of social learning (modern environments) or natural/sexual
selection upon male brains (past environments). I found the following
stuff very interesting, and would like to hear what y'all think!

Christiansen and Winkler. 1992. Hormonal, Anthropometrical, and
Behavioral Correlates of Physical Aggression in !Kung San Men of Nambia.
Aggressive Behavior 19:271-280.
The authors investigated several social, physical, and hormonal
correlates to violence (which, I admit, is only one end of a large
continuum of aggressive behavior). The !Kung San studied were those who
had been enculturated to varying degrees to the dominant, Nambian
society. Here, briefly, is what the research yielded:

--The leading causes of violence amongst the !Kung are infidelity and
(other) theft.

--Frequency of violent behavior was associated significantly with DHT and
salivary testosterone (p<.05 for each hormone). This means that men with
higher circulating male sex hormones were more prone to violence. An
exception was the non-significant association between *serum *
testosterone and aggressiveness of individuals.
Previous work suggests that testosterone in saliva ("free" or
non-bound testosterone "which is the biologically active fraction of
testosterone") is a better indicator of violence than serum
testosterone. DHT, the second most potent and biologically active
androgen, was (note) significantly associated with violence by men.

When !Kung men in the "violent" category were compared, it was
concluded that "androgen levels do correlate significantly with levels
off violent behavior."

--Bizygomatic breadth and bimammilary distance both correlated
significantly (p<.05) with aggressiveness. Big guys acted on violent
instincts more often (surprise, surprise...)
Since male body size and androgen levels correlate positively, it seems
likely that sex hormones have both direct and indirect (body size)
effects on male aggression.

--The best (or most interesting) for last: Acculturation and violent or
nonviolent behavior revealed no relationship at all between social
influence and aggressive behavior! That surprised the heck out of me,
but seems very, very relevant in our discussion about whether human
aggression is better explained as *inherently* sexually dimorphic (as
male hormone associations with violence suggest) or "socially learned," a
theory contradicted by this study.