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. If pregnancy is a risk, there may be reasons based on health. Children that result from incest between blood relatives are more likely to have birth defects.

A scenario that often happens in the context of sexual abuse of children is that a parent has sex with his or her child.

Terminology and scope 
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 is 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. 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.

Types 
The type of incest most often reported in the Western world is father-daughter incest. Incest between adults and children is a form of child sexual abuse. 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. Stepparents are  more likely to abuse stepchildren than biological parents are to abuse their children, because people typically care about their own children more than they care about their stepchildren  the Cinderella effect. 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, hence most people are never sexually attracted to their siblings or parents.

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. Among women, research by Russell (1986) and Wyatt (1985) has given estimates as high as twenty percent (for such 'incest experience'). 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.

Incest between consenting adults is very rare, or rarely reported. Willing incest between adults is a crime in most countries. It is seen by some as a victimless crime. 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.

Summary 
Most societies have some form of incest avoidance. The incest taboo is and has been one of the  most common of all cultural taboos, both in current nations and many past societies, with legal penalties imposed in some jurisdictions. Most modern societies have legal or social restrictions on closely consanguineous marriages. However, in some societies, such as that of Ancient Egypt, brother–sister, father–daughter, and mother–son relations were practiced among royalty. In addition, the Balinese and some Inuit tribes have altogether different beliefs about what constitutes illegal and immoral incest.

References 

 
Psychology