The sex drive is considered to be biological in nature. It differs from hunger and thirst in several ways. It is not a response to a lack of some substance in the body and arises from any excess of substance like hormones in the blood. The level of testosterone, the male hormone considered most responsible for the sex drive, stays relatively constant in men and the higher of male animals. Fluctuation in the level of this hormone does not seem to relate to sexual behavior in males. The same is true for the human female. In many species of lower animals–cats, for instance–there is strong connection between the presence of large amounts of estrogen, the female hormone, in the blood and sexual behavior. When these animals are “in heat”, the estrogen level in the blood is high.

          In the human beings, sexual drive is primarily triggered by external stimuli, and its expression depends very much upon learning. The stimuli that trigger it may be visual, such the sight of an attractive sexual partner, odors, etc. Sexual stimuli are also present in most popular movies, songs, plays and books. It is also woven into advertising and forms the basis of many commercials.