The Incest Taboo— — — is all about adaptation and survival

“Although we often think of birth defects when thinking of reasons for incest taboo, they’re not the drives but something else.” Petersen firmly made this claim in Introduction to Anthropology in April 2019 and I thought this was one of the mind-blowing things I learned from his lecture. It turned out I was not alone in feeling blown away, confusions appeared on others’ faces and we were impatient to have our common sense overturned by what Petersen had to say. In our culture, incest is the thing that feels naturally wrong to us. Without a second thought, we conclude that blood-related family members are not supposed to have sex because it leads to biological defects. Petersen then drew on an example by asking us to picture brothers and sisters having sex with means of birth control such as condoms, which indicates that there will be no chance of conception. Even though the odds are removed and theoretically incest is disassociated with biological defects, the taboo still evokes uncomfortable feelings in us. “It is not birth defects that make us feel this way but our culture,” Petersen stressed.

Incest taboo exists as a result of adaptation and better survival of the human race. We start discerning the argument by looking at the Martial Universe, also articulated by Petersen as “The family that does not marry out dies out.” In the diagram of Martial Universe, an inner smaller circle is labeled to present exogamy boundary and declares that one must marry outside of the nuclear family group. On the outside is the endogamy boundary that restricts that one must marry inside that boundary. Therefore, the rules of exogamy and endogamy define the universe of legitimate mates, and incest deals with the prohibition of sexual relations that don’t fall into the universe.
To understand incest, we may have to understand the evolution of sexual reproduction. First introduced by the American geneticist Hermann Joseph Muller and refined by others later, the theory “Muller’s Ratchet” provides a fundamental reason for sexual reproduction by claiming that sex “cleanses” the genome of harmful mutations. Inbreeding, however, leads to the production of less fit offspring as it reduces genetic variance in offspring and gives rise to the lethal gene. The more closely two people are related to each other, the more likely the lethal gene will become expressive. If the lethal gene becomes dominant, the offspring dies out. For instance, even when the lethal genes are recessive, one-quarter of the offspring will have the chance of dying out given both parents have recessive lethal genes.

We may notice that there is no clear notion of the incest taboo, for example, how closely related two people do we exercise the taboo? If direct family members are taken into account, why do we still see marriages between cousins in some areas? It is important to understand our reproduction from a statistics perspective. Although scientists’ research has proved that children of first cousins face a 50% higher risk of birth defects than the rest of the population, given a small inbreeding populations, these recessive lethal genes likely exist at much lower levels than a large outbreeding population. This is because the inbreeding population has a smaller pool of potential mates where offspring with a double dose of recessive lethal genes do not live long enough to pass these genes on, thus reclining the transmission of harmful alleles. On the other hand, in a large population with some degree of outbreeding, recessive lethal genes can accumulate and reach a higher level than the smaller inbreeding population.

So far, we understand there is a biological justification of incest taboo, but learned from the previous lecture, culture is a primary adaptation of modern human, hence, we need to refer to the concept in the context of our culture and society. Amongst all the explanations, a pair of contrasting theories appears to be interesting. Sigmund Freud argued that since early childhood, members of the same family naturally lust for the opposite sex parent, which is termed as Oedipus and Elektra complex. Incest taboo then was culturally invented to deal with this phenomenon. On the contrary, the Westermarck Effect suggests the opposite that the taboo itself arises naturally as products of innate attitudes that individuals who grow up together find each other unattractive as mates, such as siblings.

Taken these explanations into account, E.R. Tyler gave the best explanation we have to date about why incest taboos exist. He argued from a social perspective that people needed multiple connections which promotes a better chance of survival. Incest, however, doesn’t help build connections between this nuclear family and the outside world. Tyler’s theory is confirmed by Lee’s studies of the Ju/‘hoansi society. Lee found the true nature of marriage in the Ju is to form an important part of the system of social security. For example, if one has good relations with in-laws at different waterholes, one will never go hungry, as one can always go visit in-laws if wild food resources are given out in one’s home territory. To construct a sustainable network through marriage alliances, the Ju/‘hoansi society places strict constraints on girls and boys in whom they may or may not marry. With the strict prohibitions on kinship and name relationships, chances are largely increased of a girl or boy being married to an unrelated or distantly related spouse, which in turn, achieves the goal of expanding the family network and increases the rate of individual and group survival.

The fact that the argument challenges my common sense is another reason why it deserves a close examination. As cultivated under an industrial education system, we are accustomed to taking for granted of what sounds reasonable to us. If Petersen hadn’t raised the question, I would probably never contemplate on incest taboos within a cultural context because biological defects as one kind of answer already seem good enough. It is by looking at the contradiction — — that removing the chance of conception doesn’t stop us from feeling uncomfortable about incest — — leading to a series of deeper discussion on the causation of the phenomenon. On that account, it’s an important pedagogical method taught by Petersen that paying attention to those contradictions in our daily life may serve as a good starting point in analyzing how our society works. If mastering it, I believe the method can benefit us in all aspects of life.