Plutarch (C.E. 46–120?) was one of the earliest Western scholars interested in the incest taboo. His writings anticipated two modern theories: alliance theory and familial conflict theory. Alliance theory concludes that the incest taboo exists to create an outward reaching network of cooperative kin, which is a primary social structure essential for human survival. This network works because rules of incest force individuals to find sexual and marriage partners outside their own families. Familial conflict theory argues that incest restrictions exist to prevent destructive conflicts within the family. If family members were to engage in sexual relationships with each other, role conflicts and jealousies would destroy the effectiveness of the family institution.
The Roman historian Tacitus (C.E. 56–120) offered a theoretical framework similar to Plutarch's, suggesting alliance networks as the reason for the incest prohibitions in Roman society (Honigmann 1976). In addition to alliances, Augustine (C.E. 354–430) proposed a natural aversion to incest and an "inherent sense of decency" that prevents incestuous relationships. Thomas Aquinas (C.E. 1225–1274) advocated alliance theory and asserted that incest hindered child development. Aquinas believed that close kin marriages encourage lust and result in disruptive role conflicts that could destroy the family (Honigmann 1976).
The development of the social sciences in the nineteenth and twentieth centuries continued these themes. George Murdock (1949) and Yehudi Cohen (1978) accepted alliance theory, whereas Sigmund Freud (1950) and Talcott Parson (1954) continued the argument that incestuous relations are destructive to the family.
In The Descent of Man (1871), Charles Darwin acknowledged the family conflict model but proposed an evolutionary foundation by hypothesizing that inheritable traits allowing incest would be selected against in the evolutionary process.
Edward Westermarck, in The History of Human Marriage (1891), employed Darwinian evolutionary theory and posited that incest avoidance emerged as an instinct to prevent the genetic harm produced by inbreeding. Westermarck hypothesized that this instinct was activated when people were raised in close proximity, such as in families. He believed that this aversion would be evident most commonly among siblings, but Westermarck also proposed that sexual repugnance would develop when unrelated children were reared together. This same thesis (Westermarck's hypothesis) is currently asserted by sociobiologists of human behavior (human sociobiologists), who assume that many complex social behaviors are grounded in genetic inheritance shaped by natural selection.
Beyond these historical accounts, notable explanations of the incest taboo include demographic theory, proposed by Mariam Slater (1959) and elaborated by Charles Case (1969). It is these theorists' contention that the demographic characteristics of human breeding populations (e.g., life expectancy, birth order, and the distribution of sex among siblings) make incestuous activity in the immediate family unlikely and, at best, short-lived.
Talcott Parsons's (1954) socialization theory asserts that the incest taboo is part of a normative structure employing eroticism—and its withdrawal—as a system of sanctions in the socialization of children. The affection offered by parents and other adults (often relatives) acts as a powerful reward for "proper" behavior in children, just as its withdrawal acts as a forceful punishment. Parsons claimed that this is an effective socialization process because of the deeply social nature of the human species.
For Parsons, the incest taboo is part of the system of sexual regulations that draws a boundary beyond which the family may not wander when imparting erotic rewards. Withholding erotic rewards forces the adolescent child to participate in the larger society in order to find greater sexual fulfillment. This ties the society together through marriage and kin relationships.