comes first. We inherit our testosterone level, just as we inherit our height, body build, eye color, cholesterol level, and other characteristics. Testosterone is part of the body, the physical background out of which we think and act.
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But there is also a longer answer, which is that the effect goes both ways. Each of us has an average testosterone level, which remains about the same over the long term, but like our blood pressure, it is subject to short-term fluctuations that follow our physical and emotional ups and downs. Testosterone appears to be most sensitive to success and failure in dealing with other people, which is consistent with the fact that it evolved out of struggles for dominance among our ancestors.
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Researchers have learned a great deal about testosterone changes in people in recent years, although animal studies began around 1960. In early experiments with rats and mice, scientists found that male aggression was related to testosterone and could be reduced by castration. This work was extended to monkeys, where it was found that testosterone led to success in fighting, and that success in fighting led to increases in testosterone. As I learned from psychologist Irwin Bernstein at the Yerkes field station, when two male monkeys fight, testosterone rises in the winner and drops in the loser. 20 It is not clear why this happens, but it probably helps get the animal into the frame of mind to pick the right opponent for his next fight. When he has won, he should know he is in a good position to fight someone as strong or even stronger next time. When he has lost, he should lower his expectations and fight a weaker monkey next time. Testosterone in some female animals is also related to fighting, 21 but there has been no research on testosterone changes associated with winning and losing in females.
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Strong competition, where much is at stake, can change the body, and in many animals the change is dramatic. Among cichlid fish in Lake Tanganyika in Africa, males who defeat their competitors produce a bright coat, more brain cells, and more testosterone and sperm. Males who are defeated become submissive. Their coat becomes drab, their brain shrinks, their testicles wither, and they swim away into ignominy. 22 When male snakes fight, the outcome affects them greatly. Male rattlesnakes fight by rearing up, entwining their necks, and forcing their opponents to the ground, in a kind of disembodied arm wrestling. 23 The loser avoids further fighting, slinks away, and hesitates
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