She added that Lloyd "meticulously considers the theoretical and empirical bases for each account and ultimately concludes that there is little evidence to support an adaptionist account of female orgasm" and that Lloyd instead "views female orgasm as an ontogenetic leftover; women have orgasms because the urogenital neurophysiology for orgasm is so strongly selected for in males that this developmental blueprint gets expressed in females without affecting fitness, just as males have nipples that serve no fitness-related function".A 2005 twin study found that one in three women reported never or seldom achieving orgasm during sexual intercourse, and only one in ten always orgasmed. This variation in ability to orgasm, generally thought to be psychosocial, was found to be 34% to 45% genetic. The study, examining 4000 women, was published in Biology Letters, a Royal Society journal. Elisabeth Lloyd has cited this as evidence for the notion that female orgasm is not adaptive.Miller, Hrdy, Helen OConnell and Natalie Angier have criticized the "female orgasm is vestigial" hypothesis as understating and devaluing the psychosocial value of the female orgasm. Hrdy stated that the hypothesis smacks of sexism. OConnell said, "It boils down to rivalry between the sexes: the idea that one sex is sexual and the other reproductive. The truth is that both are sexual and both are reproductive." OConnell used MRI technology to define the true size and shape of the clitoris, suggesting that it extends into the anterior wall of the vagina (see above).
For some women, on some occasions, these contractions begin soon after the woman reports that the orgasm has started and continue at intervals of about one second with initially increasing, and then reducing, intensity. In some instances, the series of regular contractions is followed by a few additional contractions or shudders at irregular intervals. In other cases, the woman reports having an orgasm, but no pelvic contractions are measured at all.Womens orgasms are preceded by erection of the clitoris and moistening of the opening of the vagina. Some women exhibit a sex flush, a reddening of the skin over much of the body due to increased blood flow to the skin. As a woman nears orgasm, the clitoral glans retracts under the clitoral hood, and the labia minora (inner lips) become darker. As orgasm becomes imminent, the outer third of the vagina tightens and narrows, while overall the vagina lengthens and dilates and also becomes congested from engorged soft tissue.Elsewhere in the body, myofibroblasts of the nipple-areolar complex contract, causing erection of the nipples and contraction of the areolar diameter, reaching their maximum at the start of orgasm. A woman experiences full orgasm when her uterus, vagina, anus, and pelvic muscles undergo a series of rhythmic contractions. Most women find these contractions very pleasurable. Researchers from the University Medical Center of Groningen in the Netherlands correlated the sensation of orgasm with muscular contractions occurring at a frequency of 8–13 Hz centered in the pelvis and measured in the anus.