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 news stories concerning Gary Hart, Jim Bakker and, for that matter, the U.S. Marine guards at the American embassy in Moscow.

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?

A canvass of sex researchers reveals a surprising agreement that the answers to most of these questions are still unclear. Most authorities say that 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."

The Role of Hormones

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," says Davidson, "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 both 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.

The man found that deep in the solitude of his jungle research, his beard growth slowed significantly until the day before the planned visit to town. Then it speeded up enormously. After he had been in town for a few days of regular sex, it settled back to a moderate rate of growth, slowing still further after the return to the jungle.

"Anticipation turns the testosterone up," Reinisch says, indicating that the conscious mind can, at least sometimes, govern the hormones, not the other way around. "We also know that abstinence increases the sex drive for many men, but this experiment suggests it doesn't necessarily drive the testosterone level up. That seems to come only when the prospect of a sexual encounter is imminent."

There is additional, anecdotal evidence that

the male sex drive need have nothing to do with hormones at all. Social Factors

The castrati were men, chiefly in 18th century Europe, who were castrated before adolescence to preserve their high voices for choral singing. With no source of testosterone, their voices did not change, but, according to Scott Fraser, a specialist in sexual behavior and its history at the University of Southern California, many of them still developed a powerful interest in women, presumably because they learned the masculine sex role from their culture. From the ranks of the castrati, according to Fraser, came "some of the greatest philanderers in history."

"In fact, these guys often enjoyed terrific access to women because most men assumed that they would have no interest in sex and were safe with their wives," Fraser says. "What this is telling us is that psychological factors can be far more important than biological factors."

The significance of hormones in women has been studied far less, but there is evidence that it plays a role. A woman's testosterone level rises and falls in synchrony with her menstrual cycle, peaking at the middle of the cycle near the time of ovulation (around Day 14) and again about a week later. According to several studies, women report feeling their strongest sexual desires at mid-cycle.

Testosterone levels fluctuate regularly in men but on a daily cycle, peaking in the early morning. Experts disagree as to whether this coincides with peaks of male libido. Some challenge the notion because injections of the hormone produce little libidinal effect until after several days.

A more important hormonal effect on women has come from studies of women who have had their ovaries removed, a practice that once routinely accompanied hysterectomy, or removal of the uterus.

"Women who have lost their ovaries are very consistent in reporting a complete absence of libido despite a warm, loving relationship," says Winnifred Cutler of the private Athena Institute for Women's Wellness Research in Haverford, Pa. "These women, if they have a man, will have sex and they'll have pleasurable orgasms, but in between times they just don't develop the urge, the longing."

Cutler, a co-discoverer of the human pheromones -- bodily odors that alter the physiological functioning of others -- said that when such women are given testosterone injections, their desire returns.

"A woman's pure, raw libido depends on testosterone," Cutler says, "but her actual behavior, the choice of a particular partner, depends on other factors."

To the extent that hormones do affect the sex drive, the strength of that drive may depend partially on genetics. Just as some people inherit pancreatic glands that make too little or too much insulin, advocates of this view suggest, some may inherit testes or ovaries that make abnormal amounts of libido-strengthening hormones.

Closely related is the state of a person's autonomic nervous system, which governs a variety of unconscious bodily activities such as the heart, stomach and glands. There is some evidence that people inherit the degree of arousal or threshhold of sensitivity at which this system is normally maintained. Because this appears to be a generalized state, some authorities believe that men and women with a high level of normal arousal are also primed to respond faster to other stimuli such as dangerous situations that elicit a "fight or flight" response from the adrenal glands.

"People with low states of autonomic arousal are slow to respond to all kinds of stimuli," Fraser said. "But exactly the same stimulus -- a sexually attractive person walking by, for example -- might bring on a much stronger libidinal response in somebody with a higher state of autonomic arousal." 'Genetically Determined'?

Although there is no evidence as to whether such things can be inherited, many sexologists believe that they can. Paul H. Gebhard, former director of the Kinsey Institute, is one.

"Some persons have the need for frequent sexual expressions. Others require very little. And some persons respond quickly and violently, while others are slower and milder in their reactions," Gebhard wrote in the Encyclopedia Britannica. "While the genetic basis of these differences is unknown and while such differences are obscured by conditioning, there is no doubt that sexual capacities, like all other physiological capacities, are genetically determined."

Even strong advocates of genetic influence say that cultural factors play powerful roles in shaping and building the sex drive.

"What turns people on can be just about anything from the culture at large to the individual's own experience," Fraser says.

Fraser notes that while Western cultures have made the female breast an erotic stimulus for men, this is not so among the Eskimo and some South Pacific societies. It is well known that cultural tastes in erotica also change over the years. The amply proportioned women who were the ideal in the days of Peter Paul Rubens, for example, might not be as provocative to men raised on a visual diet of gaunt fashion models and slim-hipped centerfolds.

Still, virtually all authorities agree, men and women do develop individual preferences that may be far from the cultural norm but highly stimulating. Such stimuli extend beyond the bodily appearance of an attractive person to types of clothing, ways of behaving, the sound of a voice, bodily aromas, personality and a host of other traits.

"Anything that a human being perceives as a stimulus can come to be strongly stimulating and sexually arousing," Fraser says. "There's no way to tell how something might become a stimulus for a person. It may happen early in life. It may be late."

While such phenomena might seem also to account for homosexuality -- causing a person of the same sex to become sexually arousing -- sex researchers say the evidence does not seem to support this. The determinants of sexual preference seem to be quite distinct from the causes of the sex drive's strength.

Psychologists of the traditional behaviorist school argue that any stimulus will gain in erotic power after repeated, Pavlovian association with the pleasurable sensations of sex. Whether it's the sight of a woman's garter belt or the smell of a man's body or anything else, if the sensation is remembered as being closely associated with the highly enjoyable feelings of sex, it will gradually gain the ability to elicit strongly erotic feelings all by itself.

People whose sexual experiences are highly varied in types of partners and settings may thus come to find a great many stimuli erotic, the researchers say, while those with less experience respond to fewer stimuli.

One of the most powerful stimuli for many women, according to many authorities, is power in a man -- the access of a man to wealth and influence over others.

"Women are turned on by access to powerful men. Power and sexuality go together," says the Kinsey Institute's June Reinisch. "It does seem to be that powerful men are highly sexed. And it's certainly true, at least in America, that women are highly attracted to powerful men. Power has a stronger effect on most women than the physical attractiveness of the man." How People Evolved

With many other students of human sexuality, Reinisch sees an evolutionary explanation for this phenomenon. It is rooted in the theories of sociobiology, the study of how natural selection can influence social behavior. The theory, though relatively new, is grounded in Charles Darwin's own writings and provides an explanation for the differences in the way women and men express their sex drives.

Sociobiologists, such as Donald Symons of the University of California at Santa Barbara, contend that male-female differences in sexual behavior are the result of differences in the investment each must make to see that his or her genes are transmitted to the largest number of offspring.

A man produces millions of sperm and, it can be calculated, it is to his evolutionary advantage to inseminate as many women as possible. The man's cost per sperm is minuscule. With no further parental investment, a man may successfully pass on his genes to many children.

A woman, on the other hand, risks far more with each sexual encounter. She produces relatively few eggs and, should one be fertilized, she must invest nine months in the pregnancy and quite a few years in rearing the child. If a woman has more than one small child, rearing them often requires the aid of a man.

Thus, sociobiologists hold, men have evolved behaviors that make them promiscuous and less choosy about mates while women have evolved behaviors that lead them, before granting sexual favors, to seek a strong attachment to one man who shows signs of being a caring, sharing partner as well as having good economic potential.

Actually, Symons argues, the best evolutionary strategy for men is a compromise between promiscuity and parental investment in the offspring of at least one primary union stable enough to ensure that some offspring survive.

The behaviors influenced by genes are, of course, not consciously perceived as having genetic or evolutionary value. Rather, natural selection favors the survival of any complex of behavior-influencing genes that guides its possessor to behaviors that favor the largest number of offspring.

In deer, for example, the stag that can keep the largest harem of does and that drives off rival males does so not because he understands genetics but because those are the behaviors that come to predominate after a course of evolution. The behavior-influencing genes of that stag will recur in many offspring while the genes of the less dominating stag are more likely to die out with him.

In the same way, evolution shapes the sex drives of men and women, making them different because the role of each sex in reproduction is different.

Sarah Blaffer Hrdy, another sociobiologist, agrees with this view in general but notes that a study of the closest living relatives of human beings, the apes and monkeys, suggests there is more to female sexuality than may be evident. She says the evolutionary history of women suggests that, far from being biologically destined to practice monogamy, they have a powerful sex drive and a proclivity, necessarily camouflaged, to engage in sex with a variety of males. She cites the evidence of female anatomy (only women have an organ, the clitoris, with no other function than to give sexual pleasure) and the behavior of our primate ancestors.

Female monkeys, which Hrdy has studied in the wild, make much the same high parental investment in their offspring as do human females but are not monogamous. Instead, they may mate often with many males. Males dominate females in nearly all primate species, but females get what they want by having sex with a large number of the males.

Hrdy argues that the clitoris is designed for just such behavior, making females capable of repeated orgasms in close order. Males, by contrast, are temporarily impotent after each orgasm.

Hrdy also argues that women's sexuality evolved from such origins but that men have gone to extremes to suppress female sexuality. She cites a range of cultural practices intended to protect men's property rights in their women: from merely teaching girls to be demure to purdah (the seclusion of women by veiling and curtains common in Muslim and Hindu societies), claustration (confinement to a cloister once common in Europe), infibulation (surgical closure of the vagina, sometime using a ring, an ancient practice that persists in the Middle East and Africa) to clitoridectomy (amputation of the clitoris, a practice still common in parts of Africa).

It is as if cultures everywhere, especially in the past, saw women as having a voracious sexual appetite that required such severe measures to protect the man's primary genetic investment. Days and Years

While evolution may have shaped the sex drive in broad terms, day-to-day events also play a part. It is well known, for example, that the sex drive can be temporarily lowered by such factors as fatigue, mental depression, too much alcohol and social stress.

Also well "known" but false is the notion that old age reduces the sex drive.

"American people have this idea that sexuality ends somewhere in one's fifties or sixties," Reinisch says. "This just isn't so. We have plenty of evidence that it commonly continues quite strong right into the seventies and eighties. The opportunity to engage in sexual activity may be considerably reduced, but the drive is there."

Reinisch said her favorite anecdote attesting to this involves Louis XIV and his queen. When he was 72 and she was 71, according to the diary of the queen's confessor, she complained that Louis insisted upon having sex twice a day. She asked her confessor if it would be wrong to deny his request once a day. Her confessor is said to have replied, "What is good for the king is good for France."

What is good for anyone, most sex researchers agree, is a sex drive of any intensity so long as it approximates that of one's sexual partner.

"For some people, sex three times a day is the norm and it's what makes them happy. For others it's twice a year, and that can be fine, too," Reinisch says. "There is no rule. There's no good or bad about this."

For all the science that has been applied to sex, the nature of desire, when it comes down to each individual's life, is as elusive as ever and still very much the stuff of poets.