Genetic sexual attraction
People tend to select mates that are like themselves; this 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 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. These data are 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, the Stübings, fought their country's anti-incest laws. They grew up separately, met as adults, and have had four children. 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.
- The theme is present in Oedipus Rex, the Athenian tragedy by Sophocles, that was first performed c. 429 BC.
- It is also present in Le Morte d'Arthur, where King Arthur has an affair with Morgause, who he didn't know at the time was his sister. His (second) illegitimate son Mordred is born of that affair. In the last part of the Morte, Arthur and Mordred kill each other in battle.
- It is also present in Die Walküre the second of four operas in the Der Ring des Nibelungen cycle by Richard Wagner, in which separated twins Siegmunde and Sieglinde meet in adulthood and fall passionately in love. The fate of their offspring Siegfried forms the subject matter of the third opera in the cycle.
- In the German medieval tale of Gregorius, the eponymous hero is born out of incest between his parents who were twins. When he returns to his mother's country 17 years later he falls in love and marries her having no idea of the fact that he is her son.
- Vladimir Nabokov's novel Ada tells the story of an affair between siblings who believe they are merely cousins (relations between first cousins being acceptable and even common in many eras), including a love triangle with the eponymous character's believed full, actually half sister.
- The eponymous heroine of Moll Flanders, as an adult, unknowingly marries her half-brother.
- The single camera comedy Arrested Development features an ambiguously incestuous relationship between George-Michael Bluth and his cousin, Maeby Fünke.
- The novels Flowers in the Attic and Garden of Shadows also deal with an incestuous relationship between brother and sister, Cathy and Christopher, and their subsequent marriage.
- The Sam Shepard play Fool for Love involves a half brother and half sister who met as teenagers and are still in love ten years on.
- In Star Wars, Luke Skywalker and Leia Organa are separated twins who meet as adults. In The Empire Strikes Back, Leia kisses Luke in front of Han Solo to make him jealous. After Leia starts an actual relationship with Han, Luke informs Leia they are siblings, having learned this himself from his Jedi Mentor, Obi-Wan Kenobi.
- In J.R.R. Tolkien's Silmarillion, Túrin, the hero of the Tale of the Children of Húrin, meets his sister Nienor after having been separated from her in childhood. The two, failing to recognize each other as siblings, are attracted and marry.
- Jax Teller, the main character of Sons of Anarchy, almost ends up having sex with his half-sister Trinity when they first meet as adults, not knowing they are related.
- In the Korean film Old Boy an incestuous relationship occurs between a father and his daughter after a long period of separation.
- In episode "Father Dearest" of Law & Order: Special Victims Unit, a man posing as a sperm donor seduces young teenage girls, pretending to be their biological father. Psychiatrist Dr George Huang identifies the girls' sexual attraction to their father as Genetic Sexual Attraction.
- In William F. Buckley, Jr.'s novel The Redhunter, the main characters, deeply involved in the anti-Communism campaign of Sen. Joseph McCarthy, who thought that they were merely the offspring of two men who were best friends, later learn that they are in fact half-siblings and immediately break off their relationship, leaving unresolved feelings of guilt and longing.
- In Banana Yoshimoto's Novel NP, character Sui (at different times) maintains a relationship with her father and her half brother without knowing in the beginning they were related. After finding it out, she still goes on with both relationships.
- In Nip/Tuck Matt met his half sister and they both had an instant attraction without knowing their relationship.
- In the film Joe Dirt, the title character has sexual intercourse with a woman he believes to be his long-lost sister.
- In the book series The Mortal Instruments, the main protagonists, Jace and Clary fall in love with each other even though they think they are siblings. Though at first the example is a subversion as they only believe they are siblings, later on Clary's real brother appears to be sexually attracted to her as well.
- In George R.R. Martin's book series A Song of Ice and Fire, two major characters, Cersei and Jaime Lannister, twins, have an incestuous relationship that results in Cersei giving birth to her three children. While the two were always close, the affair began after Jaime had been away for quite some time and had come back to find his sister betrothed. The incestuous relationships of House Targaryen in the series are not strictly speaking examples of genetic sexual attraction as defined here, since members of the family were raised to understand they would most likely marry relatives.
- In "David O. Russell"'s debut film, Spanking the Monkey, an incestuous mother-son relationship is the focus of the story.
- In a third season episode of House, the team treats a married couple, Jeremy and Tracy, for similar symptoms. During treatment, the couple likens themselves to Romeo and Juliet, as Jeremy's father had forbidden him from becoming involved with Tracy, citing various reasons. When the team is able to diagnose both Jeremy and Tracy of a rare hereditary condition, it is revealed that the true reason Jeremy's father was opposed to their union was an affair he had had with Tracy's mother, which resulted in Tracy's birth, thus making them half-siblings.
- On the American television series Perception, the fourth episode of the second season, "Toxic", features two teenagers who were revealed to have been having an affair, against their parents wishes, due to the fact that, unbeknownst to them, they were half siblings. The girl being the result of an extramarital affair between her mother, and the boys father. The series main character, neuropsychiatrist Doctor Daniel Pierce, explains the situation as being the result of GSA by name, and infers that the Westermarck effect, which he does not name, would normally prevent it.
- 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". 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.
- 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 (NaN undefined NaN). "The architecture of human kin detection". Nature 445 (7129): 727–731. doi:10.1038/nature05510.
- Fessler, Daniel M.T.; Navarrete, C.David (NaN undefined NaN). "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, 3rd November 2011. Accessed 9th November 2011.
- "I am pregnant with my dad's baby and We are Madly In love with Each Other", Latest Trending News, 30th April 2012. Accessed 13th May 2012.
- "New Zealand father-daughter couple told to end incest". BNO News. 29 August 2012. Retrieved 29 August 2012.
- "House: Fools For Love Episode Recap". TV.com. Retrieved 24 May 2011.
- 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., and Littlewood, R. (1995). "Post Adoption Incest and Phenotypic Matching: Experience, Personal Meanings and Biosocial Implications". British Journal of Medical Psychology 68: 29–44.
- Kirsta, Alix (17 May 2003). "Genetic sexual attraction". The Guardian.
- "Genetic sexual attraction: Forbidden love". CBC News. 7 May 2009.
- Genetic Sexual Attraction