Beginning in adolescence and extending well into old age, most men experience sexual desire in the form of sexual thoughts, fantasies, and mental images many times a day.

The frequency of these sexual thoughts lessens gradually over a man's lifetime, paralleling a similar decline in testosterone, the hormone most critical to his sexual response. Nevertheless, sexual imagery occurs even into very old age and can be enhanced with visual and other aids that help a man become aroused, usually within seconds.

In women the pattern and appearance of sexual desire is different. Although female sexual desire also emerges first during adolescence, there's much more variety and much less predictability in women's responses compared to men's.

Some women report they have never experienced sexual desire and wonder what all the fuss is about. Even among women who experience sexual desire, it does not appear to be as strong or persistent as in men.

Especially in relationships that have lasted for decades, women rarely experience spontaneous episodes of sexual desire. When they do, it's typically in response to stimulation from their partner. In these instances desire follows sexual arousal, rather than the other way around.

As with men, women's levels of serum testosterone decline gradually during the life cycle, with a 50 percent drop between ages 25 and 50, but this does not put the sexes on an equivalent footing because men have 10 to 20 times the testosterone levels of women of the same age.

Menopause also reduces women's sexual desire, mainly because of a significant reduction in estrogen hormones as well as the common and uncomfortable symptoms that come with those changes.

Finally, men often desire sex as a goal in and of itself, just because it feels good. Though this also is true of women, though they may also have a broader agenda for sex, including important goals of intimacy and closeness.