Genetic sexual attraction
People tend to select mates that are like themselves, which is known as assortative mating. This holds both for physical appearances and mental traits. People commonly rank faces similar to their own as more attractive, trustworthy, etc. than average. However, Bereczkei (2004) attributes this in part to childhood imprinting on the opposite-sex parent. As for mental traits, one study found a correlation of 0.403 between husbands and wives, with husbands averaging about 2 IQ points higher. The study also reported a correlation of 0.233 for extraversion and 0.235 for inconsistency (using Eysenck's Personality Inventory). A review of many previous studies found these numbers to be quite common.
Heredity produces substantial physical and mental similarity between close relatives. Shared interests and personality traits are commonly considered desirable in a mate. The heritability of these qualities is a matter of debate but estimates are that IQ is about 80% heritable, and the big five personality factors are about 50% heritable. This data is for adults in Western countries.
For the above reasons, genetic sexual attraction is presumed to occur as a consequence of genetic relatives meeting as adults, typically as a consequence of adoption. Although this is a rare consequence of adoptive reunions, the large number of adoptive reunions in recent years means that a larger number of people are affected. If a sexual relationship is entered, it is known as incest.
GSA is rare between people raised together in early childhood due to a reverse sexual imprinting known as the Westermarck effect, which desensitizes them to later close sexual attraction. It is hypothesized that this effect evolved to prevent inbreeding.
A brother and sister couple in Germany, Patrick Stübing and Susan Karolewski, fought their country's anti-incest laws. They grew up separately, met in 2000 when he was 23 and she was 15. He moved in with his mother and sister and the couple had four children which began in January 2001, the month after their mother died. Their appeal was rejected in 2008, upholding Germany's anti-incest laws.
Kathryn Harrison published a memoir in the 1990s regarding her four-year incestuous relationship with her biological father, whom she had not seen for almost 20 years prior to beginning the relationship, titled The Kiss.
A couple in South Africa who had been together for five years had a child and discovered that they are brother and sister just before their wedding. They were raised separately and met as adults in college.
At age 18, Garry Ryan left his pregnant girlfriend and moved to the United States. The daughter, Penny Lawrence, grew up and later set out to find her missing father. When they met, they "both felt an immediate sexual attraction". They then lived together as a couple and as of April 2012 were expecting their first child together.
In August 2012, a 32-year-old father and his 18-year-old daughter were convicted of incest after they admitted to having an incestuous relationship, which began in August 2010 when the girl was 16. The incest continued until May 2012 and resulted in the couple having a daughter, who was born in 2011. The teen told the court she was in love with her father and that they had been living as 'husband and wife' after meeting each other in 2010.
In popular culture
- J.R.R. Tolkien's The Silmarillion features the story of Túrin and Nienor, siblings who meet in adulthood (Túrin having been sent away from home before his sister's birth). Because of Nienor's amnesia, the two fail to recognise one another and marry, which results in Nienor's pregnancy.
- In the novel The God of Small Things by Arundhati Roy. Estha and his sister, Rahel, are twins who were separated at a young age and then reunited as adults. In the book, sexual intercourse between the two is implied but not confirmed.
- In the original Star Wars movies, Luke Skywalker and his twin sister, Princess Leia, experienced a mild case of GSA. In The Empire Strikes Back, they even shared a kiss, although Leia did this mainly to make Han Solo jealous. Upon learning they are brother and sister, they move past their initial attraction and find other love interests.
- The anime and manga series Koi Kaze tells the story of a brother and sister (ages 27 and 15, respectively) who gradually fall in love with each other when they are reunited after a ten-year separation. As their feelings grew, they faced whether to continue their relationship and face ostracism, have a platonic relationship, or take their own lives. They end up deciding to stay together and keep their relationship a secret. A similar case occurs in Please Twins!, in which tells the tale of twins who are the same age and have met previously.
- In the manga Tsumi Ni Nureta Futari, Kasumi and Yoshiki are siblings who reunite after being separated most of their lives. On their first meeting, without knowing their biological relationship, have a one-night stand. Even after learning the truth, they attempted to continue their relationship but face rejection from their mother, friends, and love rivals. At the end, Yoshiki fakes his death in order to allow him and Kasumi to run away and be together.
- In the manga True Love, Ai and Yuzuru are siblings who had been separated for 9 years following their parent' divorce. They start to fall in love and tried to resist due to their love being forbidden. After compromising to end their relationship, they make love for the first time, and Ai decided to get married. Yuzuru finds out he is not Ai's biological brother (in fact, he learns his real parents were siblings) and they get back together.
- The film Oldboy tells the story of a man who falls in love with his sister. As revenge for her death he tricks another man to fall in love with a young girl. The girl turns out to be the man's daughter, whom he has not seen since she was a child.
- In the Dollanganger series, Corrine and Christopher, the parents of Cathy and her siblings, were initially thought to have been uncle and niece who met for the first time as ages 14 and 17 respectively (in the fifth book, they were revealed to have been half-siblings). They fell in love at first sight and became lovers, which led to them being disinherited and disowned by her parents. They then ran away together, got married, and had four children.
- In the book Forbidden by Tabitha Suzuma, siblings Lochan and Maya are raising their younger siblings together, they see each other as friends and eventually fall in love. They kiss and have sexual intercourse throughout the book.
- Portuguese author Eça de Queiroz addressed the theme in some of his books. In O Primo Basílio a man returning to his hometown seduces his cousin. In Os Maias, a brother and sister separated in their early infancy, unknowingly fall in love and have sex when they meet in their adulthood. In A Tragédia da Rua das Flores, a novel considered to be a rough draft to Os Maias, the same thing happens but between mother and son.
- In the book Speaker for the Dead by Orson Scott Card, half siblings Ouanda and Miro fall in love and wish to marry for much of the book. They later discover they are siblings and Ouanda immediately moves to other attractions, with Miro eventually following.
- BBC America: Brothers and Sisters in Love
- Kirsta, Alix (17 May 2003). "Genetic sexual attraction". The Guardian.
- Penton-Voak, I.S.; et al. (Spring 1999). "Computer graphic studies of the role of facial similarity in judgements of attractiveness" (PDF). Current Psychology: Developmental, Learning, Personality, Social 18 (1): 104–117. doi:10.1007/s12144-999-1020-4.
- Watson, David; Klohnen, Eva C.; Casillas, Alex; Nus Simms, Ericka; Haig, Jeffrey; Berry, Diane S. (1 October 2004). "Match Makers and Deal Breakers: Analyses of Assortative Mating in Newlywed Couples". Journal of Personality 72 (5): 1029–1068. doi:10.1111/j.0022-3506.2004.00289.x. PMID 15335336.
- Bouchard, Thomas J. (1 August 2004). "Genetic Influence on Human Psychological Traits. A Survey". Current Directions in Psychological Science 13 (4): 148–151. doi:10.1111/j.0963-7214.2004.00295.x.
- Bob McKeown; Aziza Sindhu (May 7, 2009). "Part 2: Genetic Sexual Attraction – Part One". The Current (CBC Radio).
- Lieberman, Debra; Tooby, John; Cosmides, Leda. "The architecture of human kin detection". Nature 445 (7129): 727–731. doi:10.1038/nature05510. PMC 3581061. PMID 17301784.
- Fessler, Daniel M.T.; Navarrete, C.David. "Third-party attitudes toward sibling incest". Evolution and Human Behavior 25 (5): 277–294. doi:10.1016/j.evolhumbehav.2004.05.004.
- Kate Connolly, "Brother and sister fight Germany's incest laws", The Guardian, 27 February 2007. Accessed 20 May 2008.
- Dietmar Hipp: "Dangerous Love: German High Court Takes a Look at Incest". Der Spiegel, 11 March 2008.
- Harrison, Kathryn (1997). The Kiss. Avon Books, Inc. ISBN 0-380-73147-9.
- STEWART MACLEAN, "Engaged couple discover they are brother and sister when their parents meet just before wedding", Daily Mail, 3 November 2011. Accessed 9 November 2011.
- "Woman carries father’s baby and claims: We’re in love". The Journal (Dublin, Ireland). 23 March 2011.
- "New Zealand father-daughter couple told to end incest". BNO News. 29 August 2012. Retrieved 29 August 2012.
- Bereczkei, Tamas; et al. (2004). "Sexual imprinting in human mate choice". Proceedings of the Royal Society B: Biological Sciences 271 (1544): 1129–1134. doi:10.1098/rspb.2003.2672. PMC 1691703. PMID 15306362.
- Greenberg, M.; Littlewood, R. (1995). "Post Adoption Incest and Phenotypic Matching: Experience, Personal Meanings and Biosocial Implications". British Journal of Medical Psychology 68 (Pt. 1): 29–44. PMID 7779767.
- Kirsta, Alix (17 May 2003). "Genetic sexual attraction". The Guardian.
- "Genetic sexual attraction: Forbidden love". CBC News. 7 May 2009.
- Genetic Sexual Attraction