9/14/83, Revised 6/20/01

Incest Taboo

David A. Gershaw, Ph.D.

Incest sexual relations with one's parents, children or siblings (brothers or sisters) is forbidden in all known cultures. However, once you get outside the immediate family to include cousins, nieces and so on, the definition varies from culture to culture. The rare historical exceptions include sibling pairings of Egyptian and Hawaiian royalty to preserve the royal family line. Why is the incest taboo in the immediate family essentially universal?

One of several possible theories involves the combining of recessive genes. For recessive genes to cause a trait, the child must receive two recessive genes, one from each parent. Suppose there is a recessive gene for a negative trait in a family. It is unlikely to pair up with another recessive gene for the same trait, if family members mate with members of the general population. However, if incestual relations occur, the chances of the child having the negative trait jump to about 25%. This may have been a contributing factor to the high incidence of hemophilia a disorder in which the blood loses its ability to clot among European royalty, when cousins married each other.

Another theory about the universality of the incest taboo deals with role conflict. (A role is the expected behavior in a social position.) When you simultaneously hold two positions that require contradictory social behaviors, this leads to role conflict. This can be illustrated with the popular country song, "I'm My Own Grandpaw." (If you haven't heard it, I guess it isn't that popular.) The song tells of a widower and his son, who meet a widow and her daughter. They fall in love. However, the widower marries the daughter, while the widow marries the son. The resulting relationship is diagrammed below.

Examine the diagram carefully. Now, since the widower married the daughter of the son and the widow, the widower is now the son-in-law of the son. Remember that the children of you son-in-law (the widower) and your daughter are your grandchildren. However, the son is the child of the widower and the daughter! Therefore, the son is his own "grandpaw."

In this situation, is the son expected to behave toward the widower as his father or his son-in-law? Is the son supposed to treat the widow as his wife or his grandmother? Is the son expected to act toward the daughter as his daughter or his stepmother? All the other members in this situation will suffer similar role conflicts.

Now let's look at role conflict in an incestual relationship. Suppose a father has an incestual relationship with his daughter, which results in a child. Is the child to be treated by the father as his child or his grandchild? (The child of your daughter is your grandchild.) Should the daughter react toward her child as her child or her sibling? Since role conflict is typical of any incestual relationship, this is why it is seen as a reason for the universal incest taboo.

The last theory to be covered is the mutual assistance theory. To illustrate, we will use an example from prehistoric times. Groups traveled across the countryside foraging for food. Suppose a 10-member family crossed into the territory of a 30-member clan. This would probably result in a conflict, with the larger group winning.

However, in case the 30-member clan came into conflict with a larger group, it would be better if they could rely on the assistance of the smaller family. How could they accomplish this? If a member from each group mated, the offspring would be in common for both groups. They are very likely to combine their resources to protect the children.

If one of the spouses died, this might lead to the breakup of the mutual assistance relationship. To avoid this, the levirate and sororate systems were developed. If a husband is killed, his widow becomes the wife of one of the brothers under the levirate system. Similarly, if a wife dies, the widower will become the husband of one of her sisters under the sororate system. In this way, the relationship between the two groups will be maintained. In addition, the more groups that any group can intermarry with, the more that group can insure its physical survival.

But what if one of the sisters in the sororate system was in love with her brother and refused to marry the widower? This would threaten the very survival of the group! This might explain why many people feel very threatened by even the hint of a possible incestual act.

Each culture or subculture probably has its own explanation for its incest taboo. However, the theories discussed here seem to apply to some extent to all cultures.

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