Thursday, May 15, 2008

Testosterone and paternal investment

As modern humans moved north into environments with longer winters, women were less able to feed themselves and their children through food gathering. They thus became more dependent on food from their male partners. For men, this greatly increased the cost of having a wife and children, thus making polygyny prohibitively expensive for all but the ablest hunters.

Initially, this situation came about by men and women pushing their respective envelopes of behavioral plasticity. It may not have been the happiest of situations, but circumstances left no other choice.Over time, however, natural selection should have improved things by favoring men who were less predisposed to polygyny and more predisposed to provide for their wives and children.

How? Apparently, by lowering testosterone levels in men once they've entered a pair bond. This has been shown by findings recently presented at this year's annual meeting of the American Association of Physical Anthropologists. According to Shur et al. (2008):

"Numerous studies reveal a negative correlation between testosterone concentration and paternal care in diverse mammals including non human primates and humans. Several researchers suggest that spousal investment accounts for the lower testosterone of married men compared to unmarried men, but findings that the lowest testosterone levels are observed in married men with children implicate paternal care as particularly relevant. Thus testosterone reduction may reflect a facultative shift in male reproductive strategy from intrasexual competition and copulation to care of young.

This hypothesis was tested with wild olive baboons, among whom lactating females form close 'friendships' with their male partners.In contrast to control males, male friends experienced a decrease in testosterone level coinciding with the birth of their female friends'infants. Male friends also maintained a lower basal testosterone level than did control males during the lactation period of their female friends. Testosterone levels in male friends increased gradually corresponding with developing infant independence.

This finding may explain the marked differences in testosterone levels we see in humans, particularly between tropical and non-tropical populations. These levels seem to decrease wherever men compete less keenly for mates (because polygyny is less common) and wherever they invest more in parenting. Lowering the level of testosterone seems to lower the threshold for expression of paternal investment.

If the testosterone level has fallen in some populations because of selection for paternal investment, we should see evidence of such selection elsewhere, e.g., altered spatial distribution of testosterone receptors in the brain, more mental space dedicated to parenting behavior...."

Respecting - And Recognizing - American D.N.A.

By Michael Medved

...[T]wo respected professors of psychiatry have recently come out with challenging books that contend that those who chose to settle this country in every generation possessed crucial common traits that they passed on to their descendents. In “American Mania,” Peter C. Whybrow of U.C.L.A. argues that even in grim epochs of starvation and persecution, only a small minority ever chooses to abandon its native land and to venture across forbidding oceans to pursue the elusive dream of a better life. The tiny percentage making that choice (perhaps only 2%, even in most periods of mass immigration) represents the very essence of a self-selecting group. Compared to the Irish or Germans or Italians or Chinese or Mexicans who remained behind in the “Old Country,” the newcomers to America would naturally display a propensity for risk-taking, for restlessness, for exuberance and self-confidence –traits readily passed down to subsequent generations. Whybrow explained to the New York Times Magazine that immigrants to the United States and their descendents seemed to possess a distinctive makeup of their “dopamine receptor system – the pathway in the brain that figures centrally in boldness and novelty seeking.”

John D. Gartner of Johns Hopkins University Medical School makes a similar case for an American-specific genotype in “The Hypomanic Edge”—celebrating the frenzied energy of American life that’s impressed every visitor since Tocqueville. The United States also benefited from our tradition of limited government, with only intermittent and ineffective efforts to suppress the competitive, entrepreneurial instincts of the populace. Professor Whybrow says: “Here you have the genes and the completely unrestricted marketplace. That’s what gives us our peculiar edge.” In other words, “anything goes capitalism” reflects and sustains the influence of immigrant genetics....