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A 1999 study by the Kinsey Institute examined the definition of sex based on a 1991 random sample of 599 college students from 29 U. states; it reported that while "virtually every college student they surveyed considered penile–vaginal intercourse to be sex," and 19–20% said that anal intercourse was not sex, 60% said oral-genital contact (fellatio, cunnilingus) did not constitute having sex.Similarly, a 2003 study published in the Canadian Journal of Human Sexuality focusing on definitions of having sex and noting studies concerning university students from the United States, the United Kingdom, and Australia reported that "while the vast majority of respondents (more than 97%) in these three studies included penile–vaginal intercourse in their definition of sex, fewer (between 70% and 90%) respondents considered penile-anal intercourse to constitute having sex" and that "oral-genital behaviours were defined as sex by between 32% and 58% of respondents".Sexual intercourse may be defined by different words, including coitus, copulation, coition, or intercourse (which is typically shorthand for sexual intercourse).The term coitus is derived from the Latin word coitio or coire, meaning "a coming together or joining together" or "to go together", and it describes a variety of sexual activities under ancient Latin names, but usually refers exclusively to penile–vaginal penetration.Reproductive sexual intercourse between non-human animals is more often termed copulation, and sperm may be introduced into the female's reproductive tract in non-vaginal ways among the animals, such as by cloacal copulation.For most non-human mammals, mating and copulation occur at the point of estrus (the most fertile period of time in the female's reproductive cycle), which increases the chances of successful impregnation.Because of this, some couples may engage in the woman on top position or the coital alignment technique, a technique combining the "riding high" variation of the missionary position with pressure-counterpressure movements performed by each partner in rhythm with sexual penetration, to maximize clitoral stimulation.Anal sex involves stimulation of the anus, anal cavity, sphincter valve or rectum; it most commonly means the insertion of a man's penis into another person's rectum, but may also mean the use of sex toys or fingers to penetrate the anus, or oral sex on the anus (anilingus), or pegging.
In humans and bonobos, the female undergoes relatively concealed ovulation so that male and female partners commonly do not know whether she is fertile at any given moment.
During coitus, the partners orient their hips to allow the penis to move back and forth in the vagina to cause friction, typically without fully removing the penis.
In this way, they stimulate themselves and each other, often continuing until orgasm in either or both partners is achieved.
These copulatory reflexes are controlled by hormones and facilitated by sexual pheromones.
Unlike mammalian copulation, where copulation is a reflex controlled by hormones and influenced by pheromones, human sexual intercourse is mostly a voluntary and learned activity that is performed in order to obtain sexual rewards (i.e., pleasure or procreation).