Monday, February 21, 2011

Human mating systems and the cultural delineation of incest

I have always been interested in the differences in the delineation of incest among various cultures. For example, Pakistanis consider marriage between first cousins to be completely acceptable and sometimes even preferred. In North America and Western Europe, this would be considered incest. In pre-industrialized Japan uncle-niece marriages were common and accepted, another form of incest within Western society. Marriage or sexual relations between individuals of the immediate family (mother, father, sister, brother) is considered as incest within all cultural contexts.

This leads me to believe that there are at least two degrees of incest: absolute, involving members of the immediate or nuclear family and relative, involving members of the extended family. The processes that define absolute incest are biological. Immediate family members share half of their genetic material with their offspring. Offspring from the same parents are related to each other by half as well. Therefore, when parents-offspring and sibling reproduce the resultant offspring is related to the parents by more than 3/4 of their genome. This is problematic for two reasons:

1) The offspring produced has decreased genetic variation, reducing its ability to exist within a wide range of environmental conditions. Decreased genetic variation also leads to decreased immune response, making the offspring more vulnerable to diseases affecting his/her parents.

2) The offspring has a higher probability of inheriting genetic problems, since the probability of both parents having the same genetic problem is higher. This is the reason why offspring of incest can have debilitating physiological conditions or diseases.

The lack of fitness (by this I am referring to Darwinian fitness; the ability of an individual to survive and reproduce) of offspring of absolute incest as compared to others is the main reason for the vehement rejection of such incestuous relationships globally. Now, offsprings from the relative incest category also have lower genetic variation as compared to others. However, unlike the case of absolute incest, the level of relatedness to its parents is between 1/2 and 3/4 (depending on the familial relationship of the parents). Therefore, the probability of offspring from such relationships to develop debilitating conditions or diseases is much lower as compared to absolute incest, but higher when compared to offspring from unrelated individuals.

One more factor in the delineation of relative incest is the social mating system. In societies that are monogamous/historically monogamous or consider monogamy to be a virtue, incest tends to include relationships between members of the extended family. In polygamous (one male partners with more than one female) and polyandrous (one female partners with more than one male), incest is limited to relationships between members of the immediate family. In my opinion, this is because offspring of polygamous or polyandrous systems are less related to each other (share one parent) than offspring from monogamous relationships (share two parents). Therefore, it is very likely that relationships between individuals of the extended family within polygamous or polyandrous relationships produce offspring that are less genetically related to each other than those from monogamous systems. This means that the probability of offspring from such relationships in polygamous/polyandrous systems to develop diseases and such is lower than from offsprings in monogamous systems. Given the low cost of darwinian fitness from relationships between extended family members in such systems, theoretically speaking, there is a strong possibility that such relationships would be socially accepted. It would be interesting to see if cultures where acceptance of relative incest exists (i.e. cousin or uncle-niece marriages), are cultures with polygamous/polyandrous mating systems or cultures that were, until the advent of western modernity, polygamous/polyandrous.

I would like to point out that my post is based on basic genetic theory and anecdotal evidence of the delineation on incest within different cultural contexts. I would encourage all of my readers to take my opinion with a grain of salt as nothing posted above has been verified either through scientific experimentation or meta analysis. Also, my theory of human mating systems does not fit a wide variety of examples of both relative or absolute incest. It does not explain the cousin marriages in the case of European royalty or the prevalence of first cousin marriages in Europe before the 1900s or the insistence of sibling marriages among most ancient Egyptian dynasties