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.

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.

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