Central to her thesis is the fact that women do not routinely have orgasms during sexual intercourse.
She analyzed 32 studies, conducted over 74 years, of the frequency of female orgasm during intercourse.
When intercourse was "unassisted," that is not accompanied by stimulation of the clitoris, just a quarter of the women studied experienced orgasms often or very often during intercourse, she found.
Five to 10 percent never had orgasms. Yet many of the women became pregnant.
Dr. Lloyd's figures are lower than those of Dr. Alfred A. Kinsey, who in his 1953 book "Sexual Behavior in the Human Female" found that 39 to 47 percent of women reported that they always, or almost always, had orgasm during intercourse.
But Kinsey, Dr. Lloyd said, included orgasms assisted by clitoral stimulation.