In biological terms, the urge to reproduce is one of the strongest basic drives. Producing a new generation to ensure the continuation of the species fulfils one of the primary functions of the human body. However, in humans, sexual intercourse is not only the means by which males and females reproduce but also an outlet for emotional expression. A sexual relationship can therefore play an important role in creating and maintaining the bond between partners.
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All living organisms maintain their populations by reproduction. Most simple organisms, such as bacteria, reproduce asexually, usually by cell division, resulting in offspring that are genetically identical.
In humans, reproduction is sexual, involving the fusion of two cells: a sperm and an egg. Eggs are produced by the ovaries in women, and sperm are produced by the testes in men. These cells are normally brought into contact through sexual intercourse. If a sperm succeeds in fertilizing an egg, DNA (genetic material) from each parent combines to create a unique individual.
Sexual reproduction results in an infinite variety of offspring. The only exceptions are identical twins, which develop when a fertilized egg divides equally to produce two individuals with the same genetic make-up.
In many animals, the urge to reproduce is largely instinctive, but, in humans, many social and psychological factors also significantly influence sexuality and reproduction. Changing social attitudes towards sexual behaviour in the last few decades have enabled both men and women to express their sexual needs more freely to each other.
Sex drive varies widely between individuals of both sexes and ranges from a complete lack of sexual interest to very powerful urges. The ease with which sexual arousal can be achieved also varies between individuals.
Human beings are probably unique among animals in seeking to engage in sexual intercourse as a pleasurable experience separate from the act of reproduction. Most people have a preference for a sexual partner of the opposite sex (heterosexual). Others have a preference for members of the same sex (homosexual), and some individuals experience attraction to members of either sex (bisexual).
Reproduction depends on an egg being fertilized and embedding in the uterus of the woman. Sexual behaviour and fertility affect the likelihood that these events will occur. If a fertile man and a woman at the fertile stage of her menstrual cycle have sex without contraception, a sperm has about a 1 in 5 chance of fertilizing an egg.
With medical advances, the process of reproduction can increasingly be controlled. Couples can choose to prevent conception by using a form of contraception. On the other hand, if a couple is unable to conceive naturally, conception can be assisted in a number of different ways.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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