The topic of incest is bound to bring up heated arguments about moral, medical, psychological, social, and religious implications. While some believe that love or sexual relationship between two related people should be a taboo, there are those who believe that consenting adults should be allowed to be in a relationship and that love cannot be planned. We often see on the news about some scandalous affair between a brother and sister, father and daughter, or mother and son, and find it difficult to comprehend. So, here are some facts about incest which will help you get a better idea about the history and science behind it.

1 Charles Darwin was the first to suspect if incest could cause defects in the offspring. He married his first cousin and had ten children, three of them died as infants and another three of them were infertile. 

Like many other Victorian upper-class families and most of his relatives, Charles Darwin married within the clan. He married his cousin, Emma. Six years after his wife bore her last child, Darwin began to wonder about the dangers of inbreeding since a cousin of his, Francis Galton, pointed out the dangers of marrying within the clan. He soon became anxious about the health issues in his own children. One of his daughters, Henrietta, inherited stress-induced vomiting from him, his son Leonard was “rather slow and backward”, Horace had attacks of shuddering, sobbing, and suffered from semi-convulsive movements. George had an irregular pulse, and his much-loved daughter, Annie, died at 10.

Darwin started experimenting on plants to find out more about inbreeding. He forced the plants to self-pollinate and discovered that the next generations fared badly. The inbred plants of next generations became small, less vigorous and weaker than those that cross-pollinated, leading him to realize that “cross-fertilization is generally beneficial, and self-fertilization injurious.”(source)

2 We are usually attracted to the scent of people genetically different from us to avoid incest. 

There is a group of genes known as Major Histocompatibility Complex (MHC), which is essential for our immune system and play an important role in immunological recognition. The MHC is expressed as codominant features of both types of genes, which means the more diverse the set of MHC genes the stronger the immune system. The MHC has evolved to form peptide-MHC complexes that are dispensed in body fluids such as blood serum, saliva, and urine, and also become volatile producing the scents that are attractive to someone genetically different, and thus ensuring genetic diversity for the offspring.

The effect is more commonly seen among females who tend to find males with dissimilar MHCs more attractive than those with similar ones, thus helping avoid incest and birth defects. However, the women who are taking hormonal contraceptives are attracted to those with similar MHCs. Another interesting thing to note is that children who are brought up together, related or not, by the same set of people or parents are less likely to develop sexual attraction as well.(1, 2)

3 Almost 50 percent of reunions between siblings, or parents and their offspring, separated at birth result in sexual attraction. This phenomenon is known as “genetic sexual attraction” (GSA).

Apart from genetics, psychological factors also play a major role in attraction and it is common for people to be attracted to someone with similar interests and attitudes. Sometimes, those two people are siblings, or an offspring and parent, reuniting after being separated at birth. This phenomenon known as “genetic sexual attraction” is so common that adoption agencies warn their clients about it.

There have been many instances of brothers and sisters, fathers and daughters, or mothers and sons developing sexual attraction especially when there was no time for the development of normal sexual taboos unlike those who are together from their birth. It could also be that the emotions during the reunion might be overwhelming enough to translate into sexual attraction because all physical or emotional expression of affection that normally develops and happens over years is now condensed into just a few moments.(1, 2)

4 During the Graeco-Roman period, sibling marriages were common among all classes in Egypt. 

One of the famous relationships of Egyptians was the marriage of Cleopatra VII and her younger brother, Ptolemy XIII. Their parents, Cleopatra V and Ptolemy XII, were also brother and sister. Sibling marriage had been an old tradition among all the rulers of Ptolemaic dynasty starting from Ptolemy IV and was done in order to keep the Ptolemaic blood “pure” and “to strengthen the line of succession.” The ancient Egyptians believed that royal women carried bloodlines and so the ruler marrying a sister of half-sister was thought to be advantageous. Apart from the royalty, there are also accounts of several of the Egyptian pharaohs marrying their siblings and having children with them.(1, 2)

5 Due to excessive inbreeding, Louis XIV of France, born in 1638, is a descendant of Louis IX of France, born in 1214, in 368 ways.

Also known as Louis the Great or the Sun King, in his 12th generation of ancestors Louis XIV had a total of 1,159 ancestors and was descended 368 times from Louis IX. (The number of ancestors a person has is counted in a binary fashion. The second generation is the parents, so they count as two ancestors. The third generation contributes four ancestors, and so on.) Between the 12th and 20th centuries, there were many marriages between Capetian cousins resulting in a lot of inbreeding. Louis XIV wasn’t the only one with inbred ancestors from the Capetian dynasty. Alfonso XIII of Spain, born 1886, had only 111 distinct ancestors in the 11th generation, instead of what would be a normal 1,024.(1, 2)