THINKING CRITICALLY What Shapes Sexual Orientation

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Sexual Behavior
T
he question of where sexual orientation
comes from is a topic of intense debate
in scientific circles, on talk shows, in
Internet chat rooms, and in everyday conversations.
T H I N K I N G C R I T I C A L LY
What Shapes Sexual
Orientation?
■ What am I being asked to believe or accept?
One point of view suggests that genes dictate our sexual orientation. According to this
view, we do not learn a sexual orientation but, rather, are born with it.
■ Is there evidence available to support the claim?
In 1995, a report by a respected research group suggested that one kind of sexual orientation—namely, that of gay men—is associated with a particular gene on the X chromosome (Hu et al., 1995). This finding was not supported by later studies (Rice et al.,
1999), but a growing body of evidence from research in behavioral genetics suggests
that genes might indeed influence sexual orientation in humans (Kendler et al., 2000;
Pillard & Bailey, 1998). One study examined pairs of monozygotic male twins (whose
genes are identical), pairs of dizygotic, or nonidentical, twins (whose genes are no more
alike than those of any pair of brothers), and pairs of adopted brothers (who are genetically unrelated). To participate in this study, at least one brother in each pair had to
be gay. As it turned out, the other brother was also gay or bisexual in 52 percent of the
identical-twin pairs but in only 22 percent of the nonidentical pairs and in just 11 percent of the adoptive pairs (Bailey & Pillard, 1991). Similar findings have been reported
for male identical twins raised apart. In such cases, a shared sexual orientation cannot
be due to the effects of a shared environment (Whitam, Diamond, & Martin, 1993). The
few available studies of female sexual orientation have yielded similar results (Bailey &
Benishay, 1993; Bailey, Dunne, & Nicholas, 2000).
Other evidence for the role of biological factors in sexual orientation comes from
research on the impact of sex hormones. In adults, differences in the levels of these
hormones are not generally associated with differences in sexual orientation. However,
hormonal differences during prenatal development might be involved in the shaping
of sexual orientation (Lalumière, Blanchard, & Zucker, 2000; Lippa, 2003; Williams
et al., 2000). For example, one study found that women who had been exposed to high
levels of androgens during their fetal development were much more likely to become
lesbians than their sisters who were not similarly exposed (Meyer et al., 1995). Studies
of nonhuman animals have found that such hormonal influences alter the structure of
the hypothalamus, a brain region known to underlie some aspects of sexual functioning (Swaab & Hofman, 1995). In humans, too, prenatal exposure to hormones and
other chemicals may be responsible for anatomical differences in the hypothalamus.
These hormone-related differences are seen not only between males and females but
between heterosexual and gay men as well (Bogaert, 2003, 2006; LeVay, 1991; Savic,
Berglund, & Lindström, 2005; Swaab et al., 2001).
Finally, a biological basis for sexual orientation is suggested by the relatively weak
effects of the environment on sexual orientation. For example, the sexual orientation of
children’s caregivers has little or no effect on those children’s own orientation. Several
studies have shown that children adopted by homosexual parents are no more or less
likely to display a homosexual orientation than are children raised by heterosexual parents (Anderssen, Amlie, & Ytteroy, 2002; Bailey et al., 1995; Tasker & Golombok, 1995).
■ Can that evidence be interpreted another way?
Like all correlational data, correlations between genetics and sexual orientation are open
to alternative interpretations. As discussed in the introductory chapter, a correlation
describes the strength and direction of the relationship between two variables, but it
does not guarantee that one variable actually influences the other. Consider again the
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Chapter 8 Motivation and Emotion
A COMMITTED RELATIONSHIP, WITH
CHILDREN Like heterosexual relation-
ships, gay and lesbian relationships can
be brief and stormy or stable and long
lasting (Kurdek, 2005). These gay men are
committed to each other for the long
haul, as evidenced by their decision to
adopt two children together. The strong
role of biological factors in sexual orientation is seen in research showing that
these children’s orientation will not be influenced much, if at all, by that of their
adoptive parents (Anderssen, Amlie, &
Ytteroy, 2002; Patterson, 2004; Stacey &
Biblarz, 2001; Tasker & Golombok, 1995).
data showing that the brothers who shared the most genes were also most likely to share
a gay orientation. What they shared was probably not a “gay gene” but, rather, a set of
genes that influenced the boys’ activity levels, emotionality, aggressiveness, and the like.
One example is “gender nonconformity” in childhood, the tendency for some boys to
display “feminine” behaviors and for some girls to behave in “masculine” ways (Bailey,
Dunne, & Martin, 2000; Knafo, Iervolino, & Plomin, 2005). Such general aspects of
their temperaments or personalities—and other people’s reactions to them—could
influence the likelihood of a particular sexual orientation (Bem, 1996). In other words,
sexual orientation could arise as a reaction to the way people respond to a genetically
determined but nonsexual aspect of personality. The influence of prenatal hormone
levels could also influence sexual orientation by shaping aggressiveness or other nonsexual aspects of behavior.
It is also important to look at behavioral genetics evidence for what it can tell us
about the role of environmental factors in sexual orientation. When we read a study
showing that 52 percent of the time, both members of identical-twin pairs have a gay,
lesbian, or bisexual orientation, it is easy to ignore the fact that the sexual orientation
of the twin pair members differed in 48 percent of the cases. Viewed in this way, the
results suggest that genes do not tell the entire story of sexual orientation. In other
words, it is not determined by unlearned, genetic forces alone. As described in the chapter on biology and behavior, the brains and bodies we inherit are quite responsive to
environmental influences. In fact, the behaviors we engage in and the experiences we
have often result in physical changes in the brain and elsewhere (Wang et al., 1995).
For example, physical changes occur in the brain’s synapses as we form new memories.
So differences seen in the brains of adults with differing sexual orientations could be
the effect, not the cause, of their behavior or experiences.
■ What evidence would help to evaluate the alternatives?
Much more evidence is needed regarding the role of genes in shaping sexual orientation.
We also have a lot to learn about the extent to which genes and hormones shape physical and psychological characteristics that lead to various sexual orientations. In studying