Evolutionary scientists have never had difficulty explaining the male orgasm, closely tied as it is to reproduction.

But the Darwinian logic behind the female orgasm has remained elusive. Women can have sexual intercourse and even become pregnant - doing their part for the perpetuation of the species - without experiencing orgasm. So what is its evolutionary purpose?

Over the last four decades, scientists have come up with a variety of theories, arguing, for example, that orgasm encourages women to have sex and, therefore, reproduce or that it leads women to favor stronger and healthier men, maximizing their offspring's chances of survival.

But in a new book, Dr. Elisabeth A. Lloyd, a philosopher of science and professor of biology at Indiana University, takes on 20 leading theories and finds them wanting. The female orgasm, she argues in the book, "The Case of the Female Orgasm: Bias in the Science of Evolution," has no evolutionary function at all.

Michael Houghton for The New York Times

Dr. Elisabeth Lloyd says the female orgasm has no evolutionary function.

Rather, Dr. Lloyd says the most convincing theory is one put forward in 1979 by Dr. Donald Symons, an anthropologist.

That theory holds that female orgasms are simply artifacts - a byproduct of the parallel development of male and female embryos in the first eight or nine weeks of life.

In that early period, the nerve and tissue pathways are laid down for various reflexes, including the orgasm, Dr. Lloyd said. As development progresses, male hormones saturate the embryo, and sexuality is defined.

In boys, the penis develops, along with the potential to have orgasms and ejaculate, while "females get the nerve pathways for orgasm by initially having the same body plan."

Nipples in men are similarly vestigial, Dr. Lloyd pointed out.

While nipples in woman serve a purpose, male nipples appear to be simply left over from the initial stage of embryonic development.

The female orgasm, she said, "is for fun."

Dr. Lloyd said scientists had insisted on finding an evolutionary function for female orgasm in humans either because they were invested in believing that women's sexuality must exactly parallel that of men or because they were convinced that all traits had to be "adaptations," that is, serve an evolutionary function.

Theories of female orgasm are significant, she added, because "men's expectations about women's normal sexuality, about how women should perform, are built around these notions."

"And men are the ones who reflect back immediately to the woman whether or not she is adequate sexually," Dr. Lloyd continued.

Central to her thesis is the fact that women do not routinely have orgasms during sexual intercourse.

She analyzed 32 studies, conducted over 74 years, of the frequency of female orgasm during intercourse.

When intercourse was "unassisted," that is not accompanied by stimulation of the clitoris, just a quarter of the women studied experienced orgasms often or very often during intercourse, she found.

Five to 10 percent never had orgasms. Yet many of the women became pregnant.

Dr. Lloyd's figures are lower than those of Dr. Alfred A. Kinsey, who in his 1953 book "Sexual Behavior in the Human Female" found that 39 to 47 percent of women reported that they always, or almost always, had orgasm during intercourse.

But Kinsey, Dr. Lloyd said, included orgasms assisted by clitoral stimulation.

Dr. Lloyd said there was no doubt in her mind that the clitoris was an evolutionary adaptation, selected to create excitement, leading to sexual intercourse and then reproduction.

But, "without a link to fertility or reproduction," Dr. Lloyd said, "orgasm cannot be an adaptation."

Not everyone agrees. For example, Dr. John Alcock, a professor of biology at Arizona State University, criticized an earlier version of Dr. Lloyd's thesis, discussed in in a 1987 article by Stephen Jay Gould in the magazine Natural History.

In a phone interview, Dr. Alcock said that he had not read her new book, but that he still maintained the hypothesis that the fact that "orgasm doesn't occur every time a woman has intercourse is not evidence that it's not adaptive."

"I'm flabbergasted by the notion that orgasm has to happen every time to be adaptive," he added.

Dr. Alcock theorized that a woman might use orgasm "as an unconscious way to evaluate the quality of the male," his genetic fitness and, thus, how suitable he would be as a father for her offspring.

"Under those circumstances, you wouldn't expect her to have it every time," Dr. Alcock said.

Among the theories that Dr. Lloyd addresses in her book is one proposed in 1993, by Dr. R. Robin Baker and Dr. Mark A. Bellis, at Manchester University in England. In two papers published in the journal Animal Behaviour, they argued that female orgasm was a way of manipulating the retention of sperm by creating suction in the uterus. When a woman has an orgasm from one minute before the man ejaculates to 45 minutes after, she retains more sperm, they said.

Furthermore, they asserted, when a woman has intercourse with a man other than her regular sexual partner, she is more likely to have an orgasm in that prime time span and thus retain more sperm, presumably making conception more likely. They postulated that women seek other partners in an effort to obtain better genes for their offspring.

Dr. Lloyd said the Baker-Bellis argument was "fatally flawed because their sample size is too small."

"In one table," she said, "73 percent of the data is based on the experience of one person."

In an e-mail message recently, Dr. Baker wrote that his and Dr. Bellis's manuscript had "received intense peer review appraisal" before publication. Statisticians were among the reviewers, he said, and they noted that some sample sizes were small, "but considered that none of these were fatal to our paper."

Dr. Lloyd said that studies called into question the logic of such theories. Research by Dr. Ludwig Wildt and his colleagues at the University of Erlangen-Nuremberg in Germany in 1998, for example, found that in a healthy woman the uterus undergoes peristaltic contractions throughout the day in the absence of sexual intercourse or orgasm. This casts doubt, Dr. Lloyd argues, on the idea that the contractions of orgasm somehow affect sperm retention.

Another hypothesis, proposed in 1995 by Dr. Randy Thornhill, a professor of biology at the University of New Mexico and two colleagues, held that women were more likely to have orgasms during intercourse with men with symmetrical physical features. On the basis of earlier studies of physical attraction, Dr. Thornhill argued that symmetry might be an indicator of genetic fitness.

Dr. Lloyd, however, said those conclusions were not viable because "they only cover a minority of women, 45 percent, who say they sometimes do, and sometimes don't, have orgasm during intercourse."

"It excludes women on either end of the spectrum," she said. "The 25 percent who say they almost always have orgasm in intercourse and the 30 percent who say they rarely or never do. And that last 30 percent includes the 10 percent who say they never have orgasm under any circumstances."

In a phone interview, Dr. Thornhill said that he had not read Dr. Lloyd's book but the fact that not all women have orgasms during intercourse supports his theory.

"There will be patterns in orgasm with preferred and not preferred men," he said.

Dr. Lloyd also criticized work by Sarah Blaffer Hrdy, an emeritus professor of anthropology at the University of California, Davis, who studies primate behavior and female reproductive strategies.

Scientists have documented that orgasm occurs in some female primates; for other mammals, whether orgasm occurs remains an open question.

In the 1981 book "The Woman That Never Evolved" and in her other work, Dr. Hrdy argues that orgasm evolved in nonhuman primates as a way for the female to protect her offspring from the depredation of males.

She points out that langur monkeys have a high infant mortality rate, with 30 percent of deaths a result of babies' being killed by males who are not the fathers. Male langurs, she says, will not kill the babies of females they have mated with.

In macaques and chimpanzees, she said, females are conditioned by the pleasurable sensations of clitoral stimulation to keep copulating with multiple partners until they have an orgasm. Thus, males do not know which infants are theirs and which are not and do not attack them.

Dr. Hrdy also argues against the idea that female orgasm is an artifact of the early parallel development of male and female embryos.

"I'm convinced," she said, "that the selection of the clitoris is quite separate from that of the penis in males."

In critiquing Dr. Hrdy's view, Dr. Lloyd disputes the idea that longer periods of sexual intercourse lead to a higher incidence of orgasm, something that if it is true, may provide an evolutionary rationale for female orgasm.

But Dr. Hrdy said her work did not speak one way or another to the issue of female orgasm in humans. "My hypothesis is silent," she said.

One possibility, Dr. Hrdy said, is that orgasm in women may have been an adaptive trait in our prehuman ancestors.

"But we separated from our common primate ancestors about seven million years ago," she said.

"Perhaps the reason orgasm is so erratic is that it's phasing out," Dr. Hrdy said. "Our descendants on the starships may well wonder what all the fuss was about."

Western culture is suffused with images of women's sexuality, of women in the throes of orgasm during intercourse and seeming to reach heights of pleasure that are rare, if not impossible, for most women in everyday life.

"Accounts of our evolutionary past tell us how the various parts of our body should function," Dr. Lloyd said.

If women, she said, are told that it is "natural" to have orgasms every time they have intercourse and that orgasms will help make them pregnant, then they feel inadequate or inferior or abnormal when they do not achieve it.

"Getting the evolutionary story straight has potentially very large social and personal consequences for all women," Dr. Lloyd said. "And indirectly for men, as well."

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NEW YORK TIMES
> May 24, 2005<br> Letters
> Perspectives on Orgasm<p>

To the Editor:

Re: "A Critic Takes On the Logic of the Female Orgasm" (May 17):

How strange to think that there would be any question about the role of the female orgasm. The orgasm, male
> or female, and the less intense joys of touch are what make sex fun.<p>

The fact that the female orgasm occurs most often outside of pure intercourse is evidence that the point of the sex act is not only to reproduce. Sex allows us to explore the manifold ways in which the body may be stimulated.  It provides the thrill of creating intimacy with a partner and allows us to demonstrate affection. And it gives us pure physical pleasure.

To paraphrase Benjamin Franklin, the female orgasm is simply proof that God loves us and wants us to be happy.

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Carly Van Orman
> Washington<p>

To the Editor:

Re "Logic of the Female Orgasm": It strikes me that the need to study this concept to death is rooted in the patriarchal idea of women as sex objects and as tools to serve men's needs. The female orgasm in and of itself throws a wrench at these assumptions.
> The Times seems a bit uncomfortable deconstructing these patriarchal assumptions, because the article should have been entitled &quot;Just for Fun: The Female Orgasm.&quot; Why pull your punches?<p>

Julie Ayala
> Baltimore<p>

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To the Editor:

I find the assertion by Dr. Elisabeth A. Lloyd that the female orgasm has no adaptive evolutionary purpose to be rather dubious ("Logic of Female Orgasm"). The female orgasm can be explained as an adaptive development by any number of plausible hypotheses. Perhaps the most obvious one is that the pleasure it brings provides an incentive for the female to engage in sex, and also solidifies pair bonding. The fact that female orgasm occurs somewhat erratically may be an indication that during human evolution, being brought to orgasm was a factor by which women selected mates. (I suspect it still is.)

Daniel Asimov
> Philadelphia<br> The writer is a mathematician.

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To the Editor:

Re "Logic of Female Orgasm": The good doctor and biologist Elisabeth Lloyd may be overlooking an alternative explanation in arguing that the female orgasm has no evolutionary function.  Perhaps it can be attributed to the notion of intelligent design. The forbidden knowledge Eve gained by biting the apple may have been the location and function of her God-given clitoris.  It is enough to turn a Darwinian diehard into a proponent of intelligent design. God is good!
> &nbsp;<br> Beverly McPhail
> Houston<p>

To the Editor:

Re "Logic of Female Orgasm": Might I suggest to those who are so desperate for an evolutionary explanation for
> women's orgasms consider: &quot;If I couldn't have orgasms, I would eventually get so tense that I might kill my children?&quot;<p>

I mean that, of course, flippantly, but it's more logical than most of the mating-centered theories that have been
> suggested.<br>  
> It seems that many people are already convinced that women's orgasms must occur for men - to keep the woman available<br> or to aid in her choice of men!

An evolutionary orgasmic man-rating system! Lovely.

This despite the fact that most women I know have their orgasms when there are no men around at all, when they
> are, in fact, entirely alone.<br>  
> Amy Letter<br> Margate, Fla.

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To the Editor:

Dr. Elisabeth Lloyd ("Logic of Female Orgasm"), in rejecting the idea that female orgasm is an evolutionary adaptation, ignores an important factor.  Yes, female orgasm during sexual intercourse is far from routine. I hypothesize, however, that the man who has sufficient consideration of the woman's pleasure to engage in stimulation of the clitoris, triggering the woman's orgasm, is that much more likely to treat the woman, and her offspring, with care and consideration later on; he will probably engender more offspring with the woman, to boot.
> &nbsp;<br> But the man who ignores the woman's pleasure is that much more likely to abandon the woman and her offspring. Thus, the female orgasm is something of a psychological assessment instrument, indicating better bets as partners.

By being tied to the likelihood of ensuring the survival of offspring, the female orgasm functions as an evolutionary adaptation.
> &nbsp;<br> Dr. Mark E. Koltko-Rivera
> Winter Park, Fla.<br> The writer is the director of research at a psychological research firm.

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To the Editor:

Struggling with the evolutionary logic of female orgasms is like trying to figure out why giraffes are made the way they are.  To me giraffes and female orgasms both represent solid evidence of a brilliant, generous and fun-loving creator.

Jemima James
> West Tisbury, Mass.<p>