Evolutionary Reasons for Incest Taboos


Incest has many taboos surrounding it and most of them are related to biological reasons. This article explores the reasons for these taboos, how they have evolved over time, and how they differ across cultures.


Why do Incest Taboos Exist?

Incest is defined as sexual relations occurring between people so closely related thatthey are forbidden by law to marry each other.1 Throughout history, human societies have demonstrated an aversion toward incest. Even those living in societies without explicit legal systems exhibit a negative reaction to the practice. The disdain for incest exists to some degree across most human cultures, and because of this ubiquity, it has been pondered by anthropologists, psychologists, biologists, and academic researchers.2 Most cultures have taboos, or social disapproval, against incest, and many academics suspect that this is biological and therefore a product of evolution. Thus, those researching the reasons for the universal nature 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 marriage to one’s first cousin only on 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


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

/var/folders/13/br3b8m6j76bf0tf5pwsqwd3h0000gn/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/Polydactyly_01_Lhand_AP-1.jpgA 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 the 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, so when close relatives reproduce, it’s likelier that the offspring will inherit two copies of one such gene. Genetic repercussions of incest can include mental and physical disabilities that are harmful to the offspring. In the case that the recessive gene is dangerous or deadly, the offspring will have decreased reproductive fitness and therefore will fail to pass on 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 Therefore, decreased reproductive fitness often impedes one’s ability to pass down their genes and is not beneficial for reproduction. Things that damage reproductive ability are not beneficial to society and this helps to explain why incest taboos have developed over time. It is beneficial to societies for people to procreate with those who will increase their chances of having healthy offspring, not decrease it. 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


Evolutionary Mechanisms that Discourage Incest

Selection pressures, or the effects that evolution has on a species, have been developed to encourage the avoidance of incest. There are many different selection effects involved in incest avoidance, including the Westermarck effect and habituation.

The Westermarck Effect

Sex differences in age maturation are one product of evolution.2 Siblings will likely become reproductively viable at different times and, as a result, are 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 toward 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


When people are cared for by their parents, they develop an idea of what kind of treatment they would like from a romantic partner in the future. Therefore, someone who looks or acts similarly to one’s parent may seem more attractive.4 However, evolution has mostly kept people from actually being attracted to their parents. This is due to habituation, which is a decrease in the responsiveness one has to a stimulus they are repeatedly exposed to.5 Habituation decreases the attraction someone will have to the actual people who raised them, therefore helping to prevent incest by encouraging people to look for a sexual partner outside their family, which is called the “optimal outbreeding perspective.”6



Legal Consequences of Incest

Punishments concerning incest vary widely depending on which society you look at. For instance, in the United States, there is a punishment for incest in every state. However, depending on the severity of the incestuous act, the punishment can range from a misdemeanor to life in prison.7 The wide range of punishments is due to the varied definitions for incest that exist in each state. In some states the definition of incest includes intercourse with step siblings, but in others first cousins are not included.7 In other countries around the world, the definitions and punishments for incest vary just as widely. For example, in some countries the highest punishment for incest is the death penalty, whereas in other countries incest may be legal between two consenting adults. These legal consequences generally contribute to the taboos that surround incest and further encourage people to avoid it.


Are these taboos a result of nature or nurture?

Most mammals express a similar reluctance toward endogamy, or incest. While biological effects such as inbred depression have worked to minimize the probability of incest in nature, humans have created taboos against the act that have made the impact of this natural aversion even more powerful. This avoidance began in biology and was facilitated by culture. The mechanisms, or selection pressures, promoting exogamy have resulted in negative attitudes towards incest across the globe.2 The importance of incest taboos has increased in many places around the world through the creation of laws and cultural stigmas surrounding intercourse with a relative. Nowadays, incest taboos are often due to a mixture of nature and nurture.


Concluding Remarks

Through evolution, the negative consequences associated with incest became clear, so humans inherited a natural aversion toward the idea of having intercourse with close relatives. The taboos surrounding incest are relatively universal, which shows that they are largely a product of biological programming. However, the societal rules determining who is deemed too close of a relative are a product of culture. Incest taboos evolved for biological reasons, but have been strengthened by societal rules over time.



  1. Merriam Webster Dictionary. “Incest.” 2016. 
  2. Daly, Martin, Wilson, and Margo. “Sex, Evolution, and Behavior.” Wadsworth Publishing Company: Belmont, California, 1978. 
  3. Marcinkowska, U. M., Moore, F. R., and Rantala, M.J.” An Eperimental Test of the Westermarck Effect: Sex Differences in Inbreeding Avoidance.” Behavioral Ecology 24, 4(1), 842-845. 2013.
  4. Fraley, R.C. and Heffernan, M. E. “How Early Experiences Shape Attraction, Partner Preferences, and Attachment Dynamics.” In Bases of Adult Attachment: From Brain to Bind to Behavior. New York, USA: Springer Publishing. 2014.
  5. Allaby, M. A dictionary of ecology (5 ed.). Oxford University Press: Oxford, UK. 2015.
  6. Hazan, C. and Zayas, V. Bases of adult attachment: Linking brain, mind and behavior. Springer Science+Business Media, LLC: New York, USA. 2015.
  7. National District Attorneys Association. “Incest Statutes.” State Statutes. 2013.

Last Updated: 05 March 2019.