Without testosterone, a fetus stands no chance of becoming male, regardless of its genetic sex. It is the XY sex chromosomes of a male embryo that encourages the embryonic tissues to produce testosterone. This then starts a path of male physical differentiation. The truth is, babies start out as neuters, sexless, undifferentiated, and the default path of development from that neutral point is into the female form. So unless the embryo produces testosterone, and its tissues respond correctly to it, at certain crucial stages in its development, it will become a female, regardless of its genetic sex. 

The neutral origin of our two sexes is seen in the histological analysis of adult male and female: for example, the clitoris and penis come from the same area of embryonic tissue; and the g-spot in the vagina and the prostate gland also have their roots in the same place. 

But even the production of testosterone by a developing embryo is no guarantee of maleness, such is nature's capriciousness. There are varying degrees of adult intersex conditions which can be traced back to some failure of embryonic development. Sometimes, for example, the chromosomally male baby, distinguished from his female sisters by his XY chromosomes, has a genetic mutation which makes his tissues more or less unresponsive to his own testosterone or, more accurately, one of its derivatives called dihydrotestosterone. The results of this unhappy accident of nature are categorized by the medical profession on a scale of severity from  1 to 7: 1 being normal male, 2 being a bit feminized (probably a fertile male with perhaps a small but possibly abnormal penis), all the way through to 7, which is a syndrome known as testicular feminization, in which the XY individual, though normal by any other standard of humanity - intelligence, personality, physical dexterity, body form - has XY chromosomes but appears to be, from birth onwards, a female, and often later develops into a very feminine woman at puberty, with rounded breasts and an attractive feminine body, but with no internal female sexual organs.

But, assuming everything goes OK, a baby with XY chromosomes will develop testicles and a penis, and all the internal male bits to go with them. Later, at puberty, his testicles become active and produce testosterone, and he develops the characteristics of an adult male.

It might follow that the higher the level of testosterone in a man's body, the hornier he feels, and the more intense his efforts to get sex. Broadly, I think this is true, but it seems we need to carry the logic a bit further. High testosterone men are also more driven and competitive, and legend has it, more aggressive, though the relationships between cause and effect is a bit dubious. If you give men more testosterone, they display more of the personality characteristics they already have - for example, an unaggressive man, given testosterone, does not become aggressive. This might mean that high levels of testosterone are the result of competitive success and high levels of aggression, not the other way round. There is some evidence to support this idea.

As a man gets older, his effective level of testosterone falls, partly because his testes become less efficient, partly because his physiology becomes less effective at removing estrogen from his system. (Estrogen is produced in all men, and clogs up the molecular receptors on his cells where testosterone usually rests. One of the liver's jobs is to remove the estrogen.) This is one reason why men get less horny as they get older. Testosterone replacement therapy can deal with the problems that come with declining testosterone: depression, reduced sex drive, impotence, lower motivation. 

(By the way, many doctors don't accept that testosterone replacement therapy for older men will cure impotence. They say many older men are impotent but have normal levels of testosterone. This is false logic because first you need to know how a man's testosterone levels have changed over time - not all men are equal to start with, as you can see by the wide range of testosterone levels that are regarded as "normal"; and second, you can't just measure total testosterone - you have to asses how much is available to the man's body tissues. Often this may be radically different to the amount in the bloodstream.

 

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