Incest means sexual intercourse between people who are closely related. Usually, this means members of the same family. In many societies, it is forbidden by law and religion. Which relatives it is forbidden to have sex with depends on the law, religion and culture.

Incest is now regarded as bad for the genetic health of the offspring, and it is often forbidden by present-day religions and laws.^[1][2] The fundamental reason has to do with genetics. Incestual societies will have a high rate of birth defects, significant enough for it to be noticed even when little was known about the cause. To some extent this is a surmise, because ancient societies had no knowledge of genetics as such. But they must have noticed some connection between birth defects and the relationship between parents. We think this because widespread bans on closely related persons marrying came long before the understanding of modern genetics.

In history (as far as we know it) there were societies which allowed incestual marriages and individual acts of incest. Famously, the ancient Egyptian royal family practised incest, though not exclusively. It is thought that the frequency of early non-trauma deaths (such as Tutankhamun) was due to the effect of incest. We do know that incest was practised in their royal family. Cleopatra was married to her brothers, Ptolemy XIII and Ptolemy XIV. Roman records show that marriages between close siblings were common in Ancient Egypt.^[3][4]

So, if pregnancy is a risk, there is a reason based on health. Children that result from incest between blood relatives are more likely to have birth defects.

The exact definition of the incest relationship varies. In some societies it is immediate relations, such as parents and their children; or brothers and sisters of the same parents. It sometimes includes extended family like aunts, nephews, nieces and uncles. That way, intercourse between cousins is legally permitted in the large majority of the world, but is disliked in many countries. In some U.S. states, such as Texas, the legal definition of incest includes first cousins, but not second cousins. A person who engages in incest may be called an incestophile or incestualist.

Other societies define it much more widely, to include all "blood relatives", or all who live in the same household, or belong to the same clan.^[5] Some religions say that sex between people related by marriage is incest. For example, the Old Testament forbids sex between siblings-in-law. In South Africa, sex between a parent-in-law and child-in-law is illegal.

There are different types of incest:

• Between an adult family member, and a child. This is usually a form of child sexual abuse.^[6] It often involves a form of threat, or the adult abuses his or her position of power. The most commonly-reported form in the Western world is father-daughter incest.^[7] This form has been shown to cause one of the worst forms of childhood trauma, a trauma that often causes serious and long-term psychological damage, especially in the case of parental incest.^[8] Stepparents are more likely to abuse stepchildren than biological parents are to abuse their children,^[9] because people typically care about their own children more than they care about their stepchildren – the Cinderella effect.^[10] It is suggested that the Westermarck effect prevents a person being attracted to anyone whom he lived in the same house as during his childhood. For this reason, most people are never sexually attracted to their siblings or parents.^[11][12]
• Between children who are related. It is thought, that incest between siblings is widespread, but that it is rarely reported.^[13] If one sibling forces the other, or there is no consent, this is likely the same case as sexual abuse of children.
• Between consenting adults.Incest between consenting adults is very rare, or rarely reported.^[6] Willing incest between adults is a crime in most countries. It is seen by some as a victimless crime.^[14] A reason why it is prohibited is because severely handicapped children are often born to parents who are blood-related. Other reasons for it being illegal are religion and to try to prevent sexual predation.

The term is often used to apply to less serious sexual relationships between related people. It is difficult to say how frequent incest is, but researchers have estimated that between 10% and 15% of people have at least one "incest experience" (this means some kind of sexual relationship falling short of actual incest). Less than 2% of these involve intercourse or attempted intercourse.^[15] Among women, research by Russell (1986) and Wyatt (1985) has given estimates as high as twenty percent (for such 'incest experience').^[8] In a survey of women in San Francisco, of those who had grown up with a stepfather, 17% (1 in 6) said that he sexually abused her. In the same study, 2.5% (1 in 40) of the women said that they had been sexually abused by their father.^[16]

Most societies have some form of incest avoidance.^[17][18]

The incest taboo is and has been one of the most common of all cultural taboos, both in current nations and many past societies,^[19] with legal penalties imposed in some jurisdictions. Most modern societies have legal or social restrictions on closely consanguineous marriages.^[20] However, in some societies, such as that of Ancient Egypt, brother–sister, father–daughter, and mother–son relations were practiced among royalty.^[21][22]

Many royal houses have suffered inbreeding. This is caused in most cases by apparently natural marriages between persons who are actually closely related. Cases have appeared which are explained, and sometimes even proved, as being dues to inbreeding. The classic examples are the European royal houses (see Habsburg) and the Egyptian pharaohs.