Incest is not such a clear-cut matter as it has been made out to be over millennia of taboos. Many participants claim to have enjoyed the act and its physical and emotional consequences. It is often the result of seduction. In some cases, two consenting and fully informed adults are involved. Many types of relationships, which are defined as incestuous, are between genetically unrelated parties (a stepfather and a daughter), or between fictive kin or between classificatory kin (that belong to the same matriline or patriline). In certain societies (the American Indians or the Chinese) it is sufficient to carry the same family name (=to belong to the same clan) and marriage is forbidden. Some incest prohibitions relate to sexual acts - other to marriage. In some societies, incest is mandatory or prohibited, according to the social class (Bali). In others, the Royal House started a tradition of incestuous marriages, which were imitated by lower classes (Ancient Egypt). The list is long and it serves to demonstrate the diversity of this most universal taboo. Generally put, we can say that a prohibition to have sex with or marry a related person should be classified as an incest prohibition, no matter the nature of the relationship.
Perhaps the strongest feature of incest has been hitherto downplayed: that it is, essentially, an autoerotic act. Having sex with a first-degree blood relative is like having sex with yourself. It is a Narcissistic act and like all acts Narcissistic, it involves the objectification of the partner. The incestuous Narcissist over-values and then devalues his sexual partner. He is devoid of empathy (cannot see the other's point of view or put himself in her shoes). For an in depth treatment of Narcissism and its psychosexual dimension, see: "Malignant Self Love - Narcissism Revisited" and "Frequently Asked Questions" (scroll down for a complete list of FAQs).
But incest involves more than a manifestation of a personality disorder or of a paraphilia (incest is considered by many to be a class of pedophilia). It harks back to the very nature of the family. It is closely entangled with its functions and with its contribution to the development of the individual within it.
A family is a mechanism of allocation of genetic and materialistic wealth. Worldly goods are passed on from one generation to the next through succession, inheritance and residence. Genetic material is handed down through the sexual act. It is the mandate of the family to increase both, either by accumulating property or by exogamy (marrying outside the family). Clearly, incest prevents both. It preserves a limited genetic pool and makes an increase of material possessions through intermarriage all but impossible.
Once allocated, the family is an efficient venue of transferring material wealth, as well as transmitting information and messages horizontally (among family members) and vertically (down the generations). A large part of the process of socialization still rides on the back of this property of the family. It is still by far the most heavyweight agent of socialization. Gender roles, for instance, are learned, emulated and assimilated mainly through the family. Incest, in itself, isolated from its social context and judgement, should not have affected this function in particular. There is no logical reason why incest should interfere with socialization, role learning or with the allocation of material resources (except, perhaps, when it comes to inheritance). Paradoxically, it is the reaction of society that transforms incest into such a disruptive phenomenon. The condemnation, the horror, the revulsion and the social sanctions distort the internal processes of the incestuous family. It is from society that the child learns that...