The behavior of simple animals consists only of reflex responses to external stimuli. However, the behavior of more complex species is also determined by the motivational system of drives and rewards within the animal. In humans, conscious awareness of our internal feelings and desires, and an understanding of our surroundings, allow us to choose the way we behave. This is true of sexual as well as of other kinds of behavior. From the reflex behavior of the simplest animals, a system of sexual drives and rewards has developed in higher animals, in humans, this system is elaborated further to encompass the pleasures we give and receive in family and social relations.

In the animal world the prime function of a sexual relationship is reproduction. Usually copulation occurs only when the female is producing eggs (ovulating) & therefore capable of becoming pregnant. Only at this time of estrous is she receptive and attractive to the male. Changes in her behavior, appearance or scent may stimulate the sexual drive of the male so that mating follows. In some species estrous occurs at regular intervals throughout the year, in others there may be one or possibly to breeding seasons in the year.

Many species of primates- a group including monkeys, apes & humans- are unusual among animals because they are able to breed at any time of the year. Further, the females may be willing to mate at times when they are not ovulating. In these species sexual activity has achieved an importance beyond reproduction, within a group of animals serving both to unite pairs of animals in sexual relationships and to maintain the structure of the group as a whole. In the human situation, sexual relations form the basis of very strong and often permanent bonds.

This bond, which we call love, is basically a very simple system of exchanging rewards. In a happy family, children absorb the idea of love and the elements of the system from their parents, learning the important signals of touch, body warmth, facial expression and words, which convey feeling and concern and establish stable loving relationships between individuals. Well-loved children, practiced in relating to each other and their parents, find it relatively easy to establish sensible relationships with others outside the family circle. During adolescence, when sex begins to permeate their lives, they come to relate readily to members of the opposite sex. The foundations of psychological well being are laid in early childhood. However, the social inhibition that surrounds the discussion of sexual matters sometimes causes minor misunderstandings about sex to develop into serious sexual problems.

In many animals, including humans, mating is preceded by courtship. In order to mate, one animal must approach another, and so it is essential that sexual approach be distinguished from aggressive approach. Courtship serves the function of informing both individuals that their proximity is not dangerous, as well as preparing each for copulation. Since sex and aggression both involve close body contact and strong physical activity (atleast in the male), it is not uncommon for courtship procedures to break down, and for one to be misinterpreted as the other. Sexual approach may be interpreted as aggressive, and result in fear and avoidance. Alternatively, aggression may be disguised as sex, as in rape and perversion. However, in the normal healthy situation, there is no connection between the motives of sex and aggression. Sexual drives, like hunger, are determined by conditions within the body, though often triggered into action by eternal stimuli. Just why we feel strong sexual needs at some times but not at others is an important question which, for one reason or another, has never been properly answered. Leavels of hormones in the blood, the hypothalamus and special centers in the brain are likely to be involved, but how they interact is not known. Some women are most sexually responsive during the week or ten days following menstruation. Men often reach their highest levels of activity in spring and early summer, and more conceptions occur in summer than at other times of the year. But these psychological rules, like the social rules and taboos surrounding sex, are readily broken when external stimuli are strong, and sexual behavior can begin at any time.

Psychologists divide sexual responses into four phases-excitement, plateau, orgasmic and resolution-which follow each other in sequence. A wide range of stimuli, both mental and physical can trigger excitement. Initial stages of excitement usually involve mental stimuli, there are few fields of human endeavor in which we seem more ready to use our imaginations freely, and some can advance almost to the point of orgasm through fantasy and imagination alone. Excitement is usually built up by sight, touch, scent or sounds, and human courtship practically always involves, at an early stage, the warmth of close body contact, kissing and touching- all relics of childhood days. In both sexes the skin of the genital regions is notably sensitive to stimulation by stroking and pressure. Other erogenous zones include the earlobes, breasts and flanks, but in the right circumstances almost any contact in love-making-indeed any stimulation by the partner, involving any of the five senses-can be acutely erogenous.

As a result of stimulation, excitement causes changes in the distribution of blood about the body, mediated by the sympathetic nervous system. The penis, clitoris, labia and internal pelvic organs become engorged with blood, the penis in particular growing to full size and assuming a suitable angle for penetration. With engorgement comes increasing awareness and breasts enlarge, the nipples extend and the skin flushes with blood, its overall sensitivity to touch increases. Blood pressure and heartbeat rise. The vaginal walls secrete a thin lubricating liquid, and secretions too moisten the labia. The vagina lengthens and distends, the uterus pulling upwards out of the way. Once the organs have reached this stage of arousal, penetration is usually easy, and further movement and stimulation carries excitement to the plateau phase.

Now blood vessels in the outer third of the vaginal wall become suffused with blood and the vagina contracts, tightening about the shaft of the penis. The tip of the penis expands slightly, and the testes are pulled inwards to the body. In the climax of orgasm muscle tensions which have built up all over the body relax, waves of muscular contraction propel semen from the ejaculatory ducts along the length of the penis and matching contractions shake the vaginal wall. Finally in the resolution phase, the organs return gradually to their normal condition. Detailed surveys in the United States and Britain have shown that individuals vary widely in their need for sexual activity. Many people are well able to live full and happy lives with little or o sex. Others find sexual activity essential as a means of expressing themselves emotionally and physically, and often need the release from tensions which orgasm brings. It is not unusual for young people of either sex to want two, three or more orgasms per week, cultural and psychological background determines whether they achieve satisfaction through heterosexual or homosexual contacts, love-making, petting or masturbation, or suppress their erotic feelings altogether. Though lovemaking is primarily a social activity, its reproductive function has not disappeared. Those who area most active sexually often forget in the excitement and happiness of love-making that they may bring unwanted children into a society which cannot provide humanely for them. Although frequency of desire and orgasm fall slowly through middle age, variation between individuals remains wide, and some maintain a happy sex life well into their seventies and eighties.