This kind of precise timing may also be true of many

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Childhood and Adolescence: Voyages in Development
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Childhood and Adolescence: Voyages in Development
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gered at the correct time during maturation for humans to develop speech fully. This kind of precise timing may also be true of many of our behavioral traits, including sexual orientation, and is why genetic research on sexual orientation presents problems (Murphy, 1997; Savin-Williams, 2005). Studying the genetic basis of sexual orientation is unusually complex, which is some- thing popular media often ignore when discussing genetic "causes" (Byrne & Parsons, 1993; Diamond, 2005). Scientists, in contrast, look for ways to work with such complex- ity. 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 homo- sexual or bisexual orientation, as compared to 8% of frater- nal 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 com- bination of genetics and interactive biopsychosocial sources may explain sexual orientation: It is believed that a genetic 327 twin studies Research that compares twins to determine what part of nature or nurture may have influenced behavior. predisposition toward homosexual orientation may exist and Identical twins share 1 00% of their genes. Research with identical that this predisposition is reinforced in the social context of and fraternal twins suggests that identical twins are more likely growing up in the same family. than fraternal twins to have the same sexual orientation.
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Childhood and Adolescence: Voyages in Development
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Chapter 13 / Exercise 03
Childhood and Adolescence: Voyages in Development
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328 imprinting A rapid early learning process by which a newborn establishes a behavior pattern of recog nition and attraction to another animal of its own kind or to an object identified as the parent.

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