Evolutionary biologists have postulated several hypotheses about the role, if any, of the female orgasm in the reproductive process for decades.

One well-known evolutionary theory has it that the female orgasm is a byproduct of ongoing selection on the male orgasm and ejaculation system. But a survey conducted recently by researchers at the University of Queensland challenges the byproduct theory.

Surveyors asked 1,803 pairs of opposite-sex twins and 2,287 pairs of same-sex twins, how often and how easily they reached orgasm. If female orgasm is a byproduct of evolution of male orgasm, opposite-sex twins should have similar orgasmic function.

Contrary to what the byproduct theory would predict, same-sex twins recorded genetically shared orgasmic functions (brother tended to share function with brother, or sister with sister) while no such pattern emerged in the analysis of opposite-sex twins.

This suggests that different genetic factors underlie male and female orgasmic function and that selection pressures on male orgasmic function do not act substantively on female orgasmic function, researchers Brendan Zietscha and Pekka Santtila wrote. These results challenge the byproduct theory of female orgasm.

The researchers caution that the study is not definitive and that a recent Journal of Sexual Medicine study by Zietsch found out that female orgasm might be a byproduct after all.

There have been many attempts to explain the evolutionary mystery of female orgasm. One, suggested by Desmond Morris in his 1967 popular-science book The Naked Ape, was that the female orgasm evolved to encourage physical intimacy with a male partner and helps reinforce the pair bond.

Other theories are based on the idea that the female orgasm might increase fertility. For example, a 30 percent reduction in size of the vagina could help clench onto the penis which would make it more stimulating for the male. The British biologists Baker and Bellis have suggested that the female orgasm may have an upsuck action (similar to the esophagus' ability to swallow when upside down), resulting in the retaining of favorable sperm and making conception more likely.

The observation that women tend to reach orgasm more easily when they are ovulating suggests that it is tied to increasing fertility.

Other biologists surmise that the orgasm simply serves to motivate sex, thus increasing the rate of reproduction, which would be selected for during evolution. Since males typically reach orgasms faster than females, it potentially encourages a female's desire to engage in intercourse more frequently, increasing the likelihood of conception.

Evolutionary scientist Stephen Jay Gould and other researchers have claimed that the clitoris is vestigial in females, and that female orgasm serves no particular evolutionary function. However, this theory was termed sexist and was criticized by many for ignoring the evolutionary advantages that result from successful conception.

Evolutionary biologist Robin Baker argues in Sperm Wars that occurrence and timing of orgasms are all a part of the female body's unconscious strategy to collect and retain sperm from more evolutionarily fit men. An orgasm during intercourse functions as a bypass button to a woman's natural cervical filter against sperm and pathogens.