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.