Although considerable research exploring biological factors contributing to homosex­uality has been done over the years, many of the findings are contradictory, and still no definitive scientific answers exist. The lack of clear-cut, consistent research findings is likely due in large part to the unique physiological and environmental factors underly­ing each individual’s development of sexual orientation. A behavior pattern as complex and variable as homosexuality is unlikely to be due to a single, simple biological cause. In fact, as we mentioned earlier, researchers do not appear to agree on what character­istics define where one sexual orientation begins and another ends. Some examples of these differences are detailed in the Spotlight on Research box.

Researchers often use twin studies to better understand the relative influences of social environment (nurture) and genetic makeup (nature). Identical twins originate from a single fertilized ovum that divides into two separate fetuses with identical genetic codes. Therefore, any differences between the twins must be due to environmental influ­ences. In contrast, fraternal twins occur when a woman’s ovaries release two ova and each ovum is fertilized by a different sperm cell. Because fraternal twins result from the fertilization of two separate eggs, their genetic makeup is no more alike than that of any other siblings. Physical and behavioral differences between fraternal twins may be due to genetic factors, environmental influences, or a combination of the two. When identi­cal twins are more alike (concordant) than same-sex fraternal twins in a particular trait, we can assume that the trait has a strong genetic basis. Conversely, when a trait shows a comparable degree of concordance in both types of twins, we can reasonably assume that environment is exerting the greater influence.

The most recent twin study recruited subjects from a twin registry in Australia. Over 1,500 same-sex identical and fraternal male and female twin pairs were included in this study. Each participant completed an anonymous questionnaire that addressed broad areas of sexuality, including items pertaining to sexual orientation. Using a strict criterion for determining homosexual orientation, the researchers found a concordance rate (the percentage of pairs in which both twins are homosexual) of 20% among iden­tical male twins and 0% among pairs of male fraternal twins. The corresponding con­cordance rates for female identical and fraternal pairs were 24% and 10.5%, respectively (Bailey et al., 2000). The higher concordance rates for identical twin pairs than for fra­ternal twin pairs provide strong evidence of a genetic component to sexual orientation in some individuals. Two other studies that used broader criteria for inclusion as homo­sexual reported much higher concordance rates for a homosexual orientation among male and female identical twins (52% and 48%, respectively) compared to same-sex male and female fraternal twin concordance rates of 22% and 16% (Bailey et al., 1991; Bailey et al., 1993).






















Research about sexual orientation would ideally use the same criteria for categorizing subjects as homosexual, bisexual, or heterosexual. If identical criteria were used, study results could build upon and be compared with one another. In practice, however, great variations exist in stud­ies of the characteristics of individuals included in a specific sexual orientation (Jordan-Young, 2010). I Figure 9.2 uses the Kinsey scale to show how differ­ently researchers group subjects into sexual orientations.

As seen in the figure, individuals considered heterosexual can range from exclusive and lifelong to almost equal other – and same-sex sexual attraction, behavior, and self-identity.

The range of parameters for including individuals in the bisexual/homosexual category is broader-including, in the Gastaud study, all individuals who did not have an exclusive and lifelong het­erosexual orientation. Another variable not evident in the figure is that some studies exclude individuals who iden­tify themselves as gay or lesbian from the homosexual category if they were not consistent in sexual fantasies and sexual behavior since puberty-which actually excludes "the majority of self­described lesbians and many gay men from studies on sexual orientation"

(Jordan-Young, 2010, p. 172).

When reading about research in this text and the media, it is important to keep in mind that research findings about sexual orientation are not based
on the same uniform criteria for inclusion in a gay, bisexual, or straight research group. The most important implication of these variations in sexual orientation categories is that they are often based on groupings that will show the stron­gest differences between the characteristics the researchers are studying. Consequently, differences reported between groups tend to be exaggerated (Jordan-Young, 2010).

0 1 2 3 4 5 6

Variations in research criteria for sexual orientation according to Kinsey scores


Heterosexual Ц Bi/Homosexual Homosexual

Gender nonconformity is the extent to which an individual differs from stereo­typical characteristics of masculinity or femininity during childhood. A biological pre­disposition toward homosexuality in some individuals is indicated by the strong link between adult homosexuality and gender nonconformity as a child (Bailey et al., 2000; Ellis et al., 2005). In a recent study, the researchers viewed home videos of children from infancy to 15 years of age. Without knowing the sexual orientation of the adults whose childhood videos they saw, the researchers rated the children on gender conforming and nonconforming characteristics. The results indicated that homosexual male and female adults exhibited significantly more gender nonconformity as children than did hetero­sexual adults (Rieger et al., 2008).

Research using brain scanning technology, magnetic resonance imaging (MRI), and positron emission tomography (PET) has typically found male-female differences in brain areas related to emotional expression and verbal skills. A 2008 Swedish study


used brain scanning technology to compare these areas of the brains of homosexual and heterosexual subjects.

These researchers found sex-atypical brain character­istics in the homosexual research subjects. The brain structures related to language and emotional expression were similar for gay men and heterosexual women. To a lesser extent, these areas in lesbians’ brains had simi­larities to those in straight men’s brains. These findings demonstrated differences in adult brain structure and function based on sexual orientation. The researchers concluded that "the results cannot be primarily ascribed to learned effects, and they suggest a linkage to neuro­biological entities" (Savic & Lindstrom, 2008, p. 9403).

Left – or right-handedness appears to be established before birth; when observed by ultrasound, a fetus indi­cates right – or left-handedness by thumb-sucking choice and greater movement of one arm. In a meta-analysis of studies with a combined total of almost 25,000 subjects, homosexual participants had 39% greater odds of being left-handed than did heterosexuals (Lalumiere et al.,

2000). Later research found that gay men had far greater odds of being left-handed than did lesbians (Lippa,

2003). These types of studies typically create compari­son groups that are limited to individuals at the ends of the Kinsey scale in order to increase the chance of find­ing differences (Jordan-Young, 2010).

Patterns of finger length tend to differ in males and females. Heterosexual women’s index fingers tend to be about the same length as their ring fingers, but heterosexual men’s ring fingers are often considerably longer than their index fingers. Researchers have compared these finger length patterns to lesbians’ and gay men’s finger length ratios. The various studies have produced widely mixed findings—some reported that gay men had finger length ratios more similar to those of straight women and that lesbians had ratios more similar to those of straight men, but several studies found no differences between finger length ratios between homosexual and heterosexual subjects. On bal­ance, research does not compellingly suggest that gay men and lesbians differ in finger length patterns from heterosexual men and women (Jordan-Young, 2010).