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Studies 6 & 7 into the causes of homosexual orientation

More detailed coverage of 2 studies based on
identical twins, and eye blinking response.

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This topic is continued from an earlier essay

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Study 6: Based on monozygotic (identical) twins:

These are twins that resulted from the splitting of a single fertilized egg -- the zygote -- into two separate zygotes with identical genetic structure. It can happen at any time prior to about 14 days after conception. They are commonly called "identical twins." Studying these twins, researchers can determine whether a trait such as homosexuality is determined by the environment or by genetics or by a combination of both. The technique involves the study of pairs of identical twins who were separated at birth and raised in different families without later contact. Being identical twins, they would have the same genetic structure. But being raised independently in different families (often in different states), they would mature (at least after birth) and experience totally unrelated environments, family types, family sizes, parenting methods, level of childhood molestation, etc. Fortunately, a U.S. university maintains an index of such twins who were raised separately since birth.

Schizophrenia occurs in about 1% of the adult population. In previous decades, the disorder was believed to be caused by incompetent parenting. Studies of identical twins overturned this belief. They showed that if one twin was schizophrenic, the chances of the other twin having the disorder is 65%. This shows that schizophrenia has a very strong genetic component. 1 Decades ago, autism was also blamed on the parents. Identical twin studies turned up similar results: if one twin was autistic, there was a 68% chance that the other was also autistic. 1 Similarly the cause of homosexuality has been attributed to lack of bonding between a child and the same-sex parent. This belief has been abandoned by almost all mental health professionals. However, many religious conservatives still promote this principle. Studies of identical twins have shown that if one twin is gay, the other has about a 55% probability of also being gay. Again, there is a very strong genetic component at work.

Assume as a working assumption that 5% of adults are gay. A typical study would involve:

  • The interview of perhaps 2,000 males, all of whom had separately raised monozygotic twins.

  • The identification of about 100 gays.

  • Tracking down the other twin in each case, to determine their sexual orientation.

Not a trivial task!

Two possibilities are:

  1. There is no genetic basis for sexual orientation. That is, a statement by Parents and Friends of Ex-gays is correct: "To date, all information and studies involving genetics have proven homosexuality to be environmental, not genetic." 2Then one would expect close to 5% of the second twins would also be gay.

  2. A homosexual orientation is totally caused by genes. One would then expect that between 0 and 100% of the other twin would also be gay, depending upon a property of the gene called its "penetrance" -- a type of power or effectiveness.

This type of test has been performed by various groups of researchers. The first such study found that 100% of the second twins were also gay. But this study was based on a very small sample size; the results it turned out to be a statistical fluke. Subsequent, larger, tests all reported that somewhat in excess of 50% of the second twins are also gay. This indicates that genes play a very significant role in determining sexual orientation. however, the number is not 100%. This indicates that environment plays a role in determining sexual orientation. It might be an event in the womb, or in early childhood. Other studies, explained below, show that a person's eventual sexual orientation is determined before they reach school age.

One problem with this and similar studies is that the researchers can ask what a person's orientation is, but not necessarily get a valid answer; a gay individual may be so firmly "in the closet" that they will not admit even to a stranger what their actual orientation is. Also, most studies classify subjects as either heterosexual or homosexual. A bisexual person might be involved with persons of the opposite gender and consider themselves to be heterosexual; their twin may also be bisexual but be involved with members of the same gender, and identify themselves as homosexual. Another problem is that identical twins share the same environment (their mother's womb) for the 9 months prior to birth. Thus, if there is a environmental factor which determines sexual orientation, it might work on the fetus before birth.

This series of replicated studies proves beyond any doubt that at least male homosexuality is largely determined before birth. As the American Psychological Association's statement on homosexuality in 1994-JUL asserts: "Research suggests that the homosexual orientation is in place very early in the life cycle, possibly even before birth..."

A more detailed explanation of this type of study appears elsewhere in this section.

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Study 7: Based on eye blinking inhibition:

In 2003-OCT-2, researchers at University of East London and at King's College announced the results of a study which differentiates between homosexuals and heterosexuals. They studied groups of homosexual and heterosexual men and women and found significant differences in an involuntary response to being startled with bursts of a loud noise. This is formally called "prepulse inhibition" or (PPI). Subjects were exposed to a low level noise, followed by an intense noise. Researchers measured the strength of the involuntary eye-blink responses. These data were then compared to similar strength measurements taken after exposure to a loud noise without the preceding low level noise. The lower the response, the stronger the level of inhibition.

The researchers stated that:

"The reaction of the lesbian test subjects was closer to that which would be expected among straight men. And, gay men reacted closer that of women, although to a lesser extent."

They found that the average PPI was:

  • 40% for heterosexual men.
  • 32% for gay men.
  • 13% for heterosexual women
  • 33% for lesbians.

Startle responses is known to be an involuntary response rather than learned reaction. It controlled by the limbic system, a region of the brain that also controls sexuality.

One of the researchers, Qazi Rahman, said:

"The startle response is pre-conscious and cannot be learned...This is very strong evidence that sexual orientation may be 'hard-wired' in this region" of the brain."

The researchers claimed that:

"... this study offers the first independent evidence of a non-learned neurological basis for sexual orientation."

Dr. Rahman said:

"These findings may well affect the way we as a society deal with sexuality and the issues surrounding sexual orientation." 1 to 4

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More studies are described in the next essay

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References used:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. Matt Ridley, "Nature via nurture: Genes, experience, and what makes us human," HarperCollins, (2003). Pages 103 &104. Read reviews or order this book safely from online book store.
  2. Mike Haley, "Straight Answers: Exposing the myths and facts about homosexuality," Focus on the Family, Love Won Out series, (2000), Page 9.
  3. Matt Ridley, "Nature via nurture: Genes, experience, and what makes us human," HarperCollins, (2003). Page 159. Read reviews or order this book safely from online book store.
  4. "Statement on Homosexuality," Parents Rights Coalition, 2000-MAY-21, at:
  5. Kevin Schattenkirk, "Being gay - A choice? Debunking some popular views on homosexuality," The Daily University of Washington, 1999, at: (No longer available online)
  6. "Toward a new national discussion of homosexuality," Focus on the Family, 1998-SEP-15, at:

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Copyright 1997 to 2014 by Ontario Consultants on Religious Tolerance
Latest update: 2014-JUL-22
Author: B.A. Robinson

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