Testosterone is a hormone that is responsible for the development and maintenance of masculine characteristics. Testosterone is released into the bloodstream by the testes (testicles) in males and to a lesser extent, by the adrenal cortex and ovaries in females. Not only does testosterone influence the growth and development of masculine physical characteristics, such as the penis and the beard, but testosterone is also related to masculine psychological characteristics and social behaviors, including aggression, power, sexual behavior, and social dominance. In addition, social experiences such as competition can cause testosterone levels to rise or fall.
Background and History of Testosterone Research
The history of testosterone research dates back to ancient times, when farmers observed that castrated animals (animals whose testes had been removed) were not very aggressive and had low sex drives. Castrated humans showed similar changes in behavior.
In 1849, German scientist Arnold Berthold conducted the first formal experiment involving testosterone. It was already known that when chickens were castrated during development, they became more docile than normal roosters. These castrated chickens, called capons, did not fight with others and did not show normal mating behavior. But when Berthold implanted testes from other birds into the abdomen of these capons, they developed into normal roosters. Berthold concluded that the testes must influence aggression and sexual behavior by releasing a substance into the bloodstream.
In 1935, Dutch researchers identified this substance, a hormone that they named testosterone. Later that same year, another group of researchers developed a method for producing testosterone from cholesterol. Through the development, in the 1960s, of a method called radioimmunoassay, researchers were able to measure the amount of testosterone circulating in the bloodstream, and shortly after that, a technique was developed to measure testosterone levels in saliva. The ability to measure testosterone levels through saliva rather than blood has made it easier and more practical to conduct research in humans.
Testosterone exerts its effects during three different life stages: the perinatal period (which includes pregnancy and the period shortly after birth), puberty, and adulthood.
During the perinatal period, testosterone influences the development of the sexual organs (e.g., the penis). Animal studies show that high testosterone levels during the perinatal period also cause the nervous system to develop in a more male-like way and cause more masculine adult behaviors. The evidence in humans for the effects of testosterone during the perinatal period is less clear. Some human studies have actually found an effect of perinatal testosterone in females but not in males. For example, high perinatal testosterone levels in females are associated with more masculine behaviors in early childhood and with more masculine personality traits, such as sensation seeking and emotional stability, in adulthood.
Testosterone levels rise during puberty, and this rise is related to the deepening of the voice, muscle growth, facial and body hair growth, and increased sex drive. There is also evidence in animals that a rise in testosterone level at the beginning of puberty influence competitive behaviors, including aggression and dominance, although scientists are not sure whether this relationship exists in humans.
Across a number of animal species, high testosterone concentrations in adult males are associated with high sex drive, and seasonal rises in testosterone (e.g., during the breeding season) are related to an increase in sexual behaviors. In humans, testosterone increases sex drive and sexual behaviors among men with abnormally low levels of testosterone but not among men who already have testosterone levels within the normal range.
Adult levels of testosterone are also associated with aggression and competition over food and mates. In animals, seasonal rises in testosterone (e.g., during the breeding season) are associated with increases in aggression and mate competition. There is also a small relationship between testosterone and aggression in humans. For example, several studies of male and female prisoners have found that prisoners with higher testosterone commit more violent crimes and break more rules in prison.
Testosterone is also related to power and social dominance. Animals with high testosterone levels tend to have high social rank within status hierarchies. In humans, high testosterone levels are associated with more masculine, dominant facial characteristics and with personality styles that are related to power and dominance. In addition, individuals with high testosterone levels are more reactive to and pay more attention to threats to their status, such as losing in a competition. For example, one study found that individuals with high testosterone levels felt badly (e.g., irritable, hostile) and could not concentrate after they lost in a competition but felt fine and could concentrate quite well after they won in a competition.
Changes in Testosterone Levels
Testosterone levels can change in both the short term and the long term. In humans, testosterone levels peak in the late teens to early 20s and decline slowly but steadily after that. Testosterone levels also change throughout the day: They are highest in the morning, drop over the course of the day, and rise again in the evening. In a number of animal species, there are seasonal changes in testosterone, and testosterone levels are typically highest during the breeding season. Social experiences can also cause testosterone levels to change. In animals and in humans, winners of competitions tend to increase in testosterone, whereas losers tend to drop in testosterone. In addition, testosterone can increase in response to sexual stimuli, such as the presence of a female.
Gender Differences in Testosterone
Most of the research on testosterone has focused on men, but more studies have begun examining women. Although women have only about one-seventh the testosterone levels as men, testosterone still seems to play a role in women. For example, research has found that women with higher testosterone levels tend to smile less often, score higher on tests of social dominance, and are more vulnerable to stereotype threat.