A there is so much individual and gender variation in

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A: There is so much individual and gender variation in sexual attraction and behavior, even within families, and there is enough evidence from studies of identical twins to suggest that internal mechanisms do influence sexual orientation. But researchers don't have evidence yet to say that a brain mechanism produces sexual orientation. Society plays a role, too, and perhaps in time we will see how nature and nurture both contribute. Studying the genetic basis of sexual orientation is unusually complex, which is something popular media often ignore when discussing genetic “causes” (Byrne & Parsons, 1993; Diamond, 2005). Scientists, in contrast, look for ways to work with such complexity. They have found that studies of twins are an important way to research the complex effect of genes on behavior. Twin studies compare twins to see what part of nature or nurture may influence behavior. There are two types of twins: monozygotic (MZ), or “identical,” twins share 100% of their genes. This means, then, that the variation that occurs in the traits of MZ twins, whether they be in weight, intelligence, depression, or sexual orientation, is the result of their individual life experiences, or the influence of nurture. Dizygotic (DZ), or “fraternal,” twins share only 50% of their genes, which is the same as siblings who are not twins. Because MZ twins are more similar than DZ twins, it is possible to tease out the effects of genes, culture, and unique experiences when it comes to sexual orientation (Bailey & Pilliard, 1991). Research shows that if one identical twin is gay, the other is most likely to be gay, too; or if one is straight, the other is most likely to be straight (Blanchard & Bogaert, 2004). This research may allow us to see the likelihood that MZ brothers have the same sexual orientation compared to DZ brothers or non-twin brothers. The largest twin studies have determined that 30% of identical twins share a homosexual or bisexual orientation, as compared to 8% of fraternal twins. Note that this 30% linkage is not even close to 100%, though identical twins share 100% of their DNA. This means that 70% of the individuals' source of sexual orientation is social. The social factors may include social learning experiences, family environment, and culture. A twin study is an excellent example of how the combination of genetics and interactive biopsychosocial sources may explain sexual orientation: It is believed that a genetic predisposition toward homosexual orientation may exist and that this predisposition is reinforced in the social context of growing up in the same family.
Identical twins share 100% of their genes. Research with identical and fraternal twins suggests that identical twins are more likely than fraternal twins to have the same sexual orientation.

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