Evolutionary Reasons for Incest Taboos

Why do Incest Taboos Exist?

Incest is defined as sexual relations occurring between people so closely related that they are forbidden by law to marry each other.1 Throughout history, human societies have demonstrated an aversion towards incest. Even those living in societies without explicit legal systems exhibit a negative reaction to the idea of incest. The disdain for incest exists to some degree across most human cultures,2 and because of this ubiquity, it has been pondered by anthropologists, psychologists, biologists, and academic researchers. Because human cultures are so varied and exhibit so many different features, the fact that the majority of human societies disapprove of incest is one of curiosity. Most cultures have taboos, or social disapproval, against incest, and many academics suspect that this is biological and therefore a product of evolution. If something was evolved, then there must have been biological forces working towards its perpetuation. Thus, those researching the reasons for the universality of incest taboos have attempted to uncover the biological reasons for this phenomenon. 

How is incest treated across cultures?

Taboos against sexual relations among mother and son, father and daughter, and brother and sister exist in almost every culture.2 This makes sense, as these pairings are the most closely related biologically. Rules concerning reproduction with one’s kin (family) may vary in scope. For example, some societies find distant relatives to be acceptable mates, and therefore the taboo rests on the society’s definition of one’s kin group. Marriage to one’s first cousin is seen as acceptable in some societies, while others forbid it. Still, others permit only marriage to one’s first cousin on only the mother’s or the father’s side, and thus differentiate between type of first cousin. Exogamy, or reproduction outside of one’s nuclear family, is encouraged in all human cultures.2 Additionally, most mammals express a similar reluctance toward endogamy, or incest. 


Are these taboos beneficial to society? Why did they evolve?

A tendency towards exogamy, if present in most non-human mammals, must have some biological basis. If something has evolved, it’s likely that it increases the reproductive fitness or ability to create viable offspring of the individuals who possess this trait. This can be seen in the biological occurrence of inbreeding depression.2 Recessive traits are hidden in people with only one copy of a gene. When close relatives reproduce, it’s likelier that the offspring will inherit two copies of one such gene. In the case that the recessive gene is dangerous or deadly, the offspring will have decreased reproductive fitness and therefore will fail to perpetuate their genes. If reproduction does not occur, the individual’s genetic lineage will cease to exist and these traits will be removed from the evolutionary gene pool.2 These taboos have a clear biological validity, which has caused many cultures to create institutions such as marriage around this natural aversion. Incest taboos have played an important part in shaping society, as kin relations are a major facet of most cultures. Incest taboos function to promote societal organization, with the construction of cultural practices such as marriage based on this ingrained characteristic.2


What mechanisms exist to prevent incest?

Selection pressures, or the effects that evolution has on a species, have been developed to encourage the avoidance of incest. Sex differences in age maturation is one such product of evolution.2 Siblings will likely become reproductively viable at different times and are as a result less likely to reproduce. Additionally, individuals experience a lack of desire to have intercourse with those who they recognize as relatives. This becomes clear through the Westermarck effect, in which individuals who are raised together as children do not develop sexual attraction towards one another. Even if unrelated to one another, people who are raised simultaneously or by the same parent will become averse to the idea of having sex with one another.3


Are these taboos a result of nature or nurture?

While biological effects such as inbred depression have worked to minimize the probability of incest in nature, humans’ creation of taboos against the act has made the impact of this natural aversion more powerful. This is because when humans were evolving, those who were within their group and therefore raised by the same parents were extremely likely to belong to the same kin group.2 This avoidance began in biology and was facilitated by culture. The mechanisms, or selection pressures, promoting exogamy have resulted in a nearly ubiquitous negative attitude towards incest.2 Its importance in human culture can be seen in the example of stepparents. While they are not biologically related to the stepchildren, this sexual aversion has become so ingrained in culture that relations between any two members of the kin group are seen as detrimental. 

Because incestual behavior was greatly diminished during evolution, humans inherit a natural aversion to the idea of having intercourse with close relatives. The universality of this characteristic shows that it is a product of biological programming, but the societal rules determining who is deemed too close of a relative are a product of culture. Taboos against incest are shared among humans and therefore maintain societal organization across cultures, but to varying extents.


  1. http://www.merriam-webster.com/dictionary/incest. 2016. 
  2. Daly, Martin and Wilson, Margo. “Sex, Evolution, and Behavior.” Wadsworth Publishing Company. Belmont,      California, 1978. 
  3. “What is the Westermarck Effect?” wiseGEEK.com, February 2016. 

Last Updated 07 March 2016.

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