Genetic sexual attraction (GSA) refers to sexual attraction between close genetic relatives, such as siblings or cousins, who meet for the first time as adults. The term was coined in the late 1980s by Barbara Gonyo to describe her experiences helping reunited relatives in adoption support groups. GSA is attributed to factors like assortative mating preferences for similar physical and personality traits that relatives share due to their genetic similarity. However, relatives raised together from an early age are desensitized to sexual attraction due to the Westermarck effect, which may have evolved to prevent inbreeding. While rare, there are documented cases of GSA resulting in long-term relationships and children between relatives who reunited as adults.

1. Genetic sexual attraction
Genetic sexual attraction (GSA) is sexual attraction between
close relatives, such as siblings or half-siblings, a parent
and offspring, or first and second cousins, who first meet as
adults.
The term was coined in the US in the late 1980s by Barbara
Gonyo, the founder of Truth Seekers in Adoption, a Chicagobased support group for adoptees and their new-found
relatives.
Contributing factors
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 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.

2. 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.
Instances
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 up separately and
met as adults in college.
Garry Ryan at 18 left his pregnant girlfriend and he moved to
America. 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.