In addition to observing that the majority of their female subjects could only have clitoral orgasms, they found that both clitoral and vaginal orgasms had the same stages of physical response. On this basis, they argued that clitoral stimulation is the source of both kinds of orgasms, reasoning that the clitoris is stimulated during penetration by friction against its hood; their notion that this provides the clitoris with sufficient sexual stimulation has been criticized by researchers such as Elisabeth Lloyd.Australian urologist Helen OConnells 2005 research additionally indicates a connection between orgasms experienced vaginally and the clitoris, suggesting that clitoral tissue extends into the anterior wall of the vagina and that therefore clitoral and vaginal orgasms are of the same origin. Some studies, using ultrasound, have found physiological evidence of the G-spot in women who report having orgasms during vaginal intercourse, but OConnell suggests that the clitoriss interconnected relationship with the vagina is the physiological explanation for the conjectured G-spot. Having used MRI technology which enabled her to note a direct relationship between the legs or roots of the clitoris and the erectile tissue of the "clitoral bulbs" and corpora, and the distal urethra and vagina, she stated that the vaginal wall is the clitoris; that lifting the skin off the vagina on the side walls reveals the bulbs of the clitoris—triangular, crescental masses of erectile tissue.
Though Dixson classifies humans as mildly polygynous in his survey of primate sexuality, he appears to have doubts, when he writes, "One might argue that ... the females orgasm is rewarding, increases her willingness to copulate with a variety of males rather than one partner, and thus promotes sperm competition." Ryan and Jethá use this as evidence for their theory that partible paternity and promiscuity were common for early modern humans. Adaptive or vestigial
The clitoris is homologous to the penis; that is, they both develop from the same embryonic structure. While researchers such as Geoffrey Miller, Helen Fisher, Meredith Small and Sarah Blaffer Hrdy "have viewed the clitoral orgasm as a legitimate adaptation in its own right, with major implications for female sexual behavior and sexual evolution," others, such as Donald Symons and Stephen Jay Gould, have asserted that the clitoris is vestigial or nonadaptive, and that the female orgasm serves no particular evolutionary function. However, Gould acknowledged that "most female orgasms emanate from a clitoral, rather than vaginal (or some other), site" and stated that his nonadaptive belief "has been widely misunderstood as a denial of either the adaptive value of female orgasm in general, or even as a claim that female orgasms lack significance in some broader sense".