Once it was known simply as desire , and it was the stuff of poets and novelists.
In this scientific age it is the sex drive , and although researchers of many disciplines have analyzed it from every angle, it remains as elusive and mysterious as ever.
"People have been studying the sex drive for a long time now, and nobody can say exactly what makes people feel more or less sexy," says June Reinisch, director of the Kinsey Institute at Indiana University. "We do know the sex drive varies a lot from one person to the next and within the same person from one time to the next. There is no one answer. There can be many causes."
Although researchers say sex is seldom far from the consciousness of most adults, questions about it have become more prominent with the recent major news stories.
What is it that can be so powerful as to make people forget their better judgment and risk so much? Are some people more sexually driven than others? Do we inherit our level of sex drive or learn it or is it a mixture of both? Are men just as vulnerable to "raging hormonal influences" as some allege women to be? What governs changes in the sex drive from day to day, decade to decade?
Answers Still Unclear
A canvass of sex researchers reveals a surprising agreement that the answers to most of these questions are still unclear. Most authorities say the strength of the sex drive is determined by a combination of factors that differ greatly from one person to the next.
"Hormones have a lot to do with the basic strength of your sex drive, but your cultural background has a lot to do with the way you express it. That means your sex drive is a result of who you are biologically and what kind of world you live in," says Julian Davidson, a Stanford University physiologist who studies the links between male libido and male potency.
He points out that libido and potency do not necessarily go together. "A man can have a high libido, a strong sex drive, but still be impotent and unable to get an erection. Or a man can have no problems at all with potency but still not have a strong sex drive.
"Biology and culture interact in a very complex way, and it's hard to sort them out," Davidson says. "You can make up all kinds of stories to explain why a man might do the things he does. You have no way of knowing whether you're right, though."
Even when scientists examine a single factor, even one as central as sex hormones, it is hard to come up with an unambiguous explanation for their role.
The male hormone testosterone is almost universally accepted as the chief biochemical player in the sex drive. Men have a lot, produced in their testes, but women have some too, produced in their ovaries.
Animal experiments show that when testosterone is given to animals of either sex, it increases their aggressiveness and the intensity of their sexual behavior. Female rats given the hormone, for example, also begin behaving like males, making sexual displays and trying to mount other females.
"Testosterone," Davidson says, "is the only real aphrodisiac. But I don't recommend it. It has to be injected, and it's very tricky."
In humans, the effects of testosterone are harder to analyze. Giving testosterone to a man usually does not increase his sex drive unless it was abnormally low to begin with. Then, researchers say, it strengthens the drive.
The effect on women is more controversial. While some studies show that testosterone increases the sex drive (also enlarging the clitoris and causing the growth of facial hair if given long enough), others challenge the methods used because the women were said to be aware that they were receiving a sex stimulant. While the physiological effects of testosterone are undeniable, the behavioral effects, the critics point out, could have been entirely psychological.
Experiments in which male volunteers were shown pornographic movies and monitored for penile erection and testosterone levels in the blood show a strong correlation. The more erotic the movie, the stronger the erection and the higher the hormone level in the blood.
Unclear, however, is the detailed sequence of events. Were conscious fantasies about the movie enough to cause erection directly, releasing testosterone as a byproduct, or did the fantasies act first by stimulating testosterone production that, in turn, caused erection?
Some insight has come from a now classic 1970 report in the British journal Nature by a scientist whose field work required him to stay alone in the jungle for weeks at a time, coming into town periodically for supplies and sex. The man, whose report was published anonymously but which is vouched for by well-known colleagues, weighed his beard shavings every day. This was an indirect measure of his testosterone level because this is the hormone that stimulates beard growth.