Eight major studies of identical twins in Australia, the U.S., and Scandinavia during the last two decades all arrive at the same conclusion: gays were not born that way.
“At best genetics is a minor factor,” says Dr. Neil Whitehead, PhD. Whitehead worked for the New Zealand government as a scientific researcher for 24 years, then spent four years working for the United Nations and International Atomic Energy Agency. Most recently, he serves as a consultant to Japanese universities about the effects of radiation exposure. His PhD is in biochemistry and statistics.
Identical twins have the same genes or DNA. They are nurtured in equal prenatal conditions. If homosexuality is caused by genetics or prenatal conditions and one twin is gay, the co-twin should also be gay.
“Because they have identical DNA, it ought to be 100%,” Dr. Whitehead notes. But the studies reveal something else. “If an identical twin has same-sex attraction the chances the co-twin has it are only about 11% for men and 14% for women.”
Because identical twins are always genetically identical, homosexuality cannot be genetically dictated. “No-one is born gay,” he notes. “The predominant things that create homosexuality in one identical twin and not in the other have to be post-birth factors.”
Dr. Whitehead believes same-sex attraction (SSA) is caused by “non-shared factors,” things happening to one twin but not the other, or a personal response to an event by one of the twins and not the other.
For example, one twin might have exposure to pornography or sexual abuse, but not the other. One twin may interpret and respond to their family or classroom environment differently than the other. “These individual and idiosyncratic responses to random events and to common environmental factors predominate,” he says.
The first very large, reliable study of identical twins was conducted in Australia in 1991, followed by a large U.S. study about 1997. Then Australia and the U.S. conducted more twin studies in 2000, followed by several studies in Scandinavia, according to Dr. Whitehead.
“Twin registers are the foundation of modern twin studies. They are now very large, and exist in many countries. A gigantic European twin register with a projected 600,000 members is being organized, but one of the largest in use is in Australia, with more than 25,000 twins on the books.”
A significant twin study among adolescents shows an even weaker genetic correlation. In 2002 Bearman and Brueckner studied tens of thousands of adolescent students in the U.S. The same-sex attraction concordance between identical twins was only 7.7% for males and 5.3% for females—lower than the 11% and 14% in the Australian study by Bailey et al conducted in 2000.