Demonstrating Biological Influences on Individual Differences in Masculinity and Femininity: Behavior Genetic Studies - The Case for Nature

Gender, Nature, and Nurture - Richard A. Lippa 2014

Demonstrating Biological Influences on Individual Differences in Masculinity and Femininity: Behavior Genetic Studies
The Case for Nature

The evidence just summarized strongly suggests that biology contributes to three kinds of human sex differences. But remember, there is a second side to gender: individual differences in masculinity and femininity. Correlational research on sex hormones and human behavior provides information about biological influences on masculinity and femininity; it also offers hints about biological influences on sex differences. For example, when studies show that high-testosterone men are more aggressive, risk-taking, and nonverbaily dour than low-testosterone men, they indirectly suggest that testosterone is a biological factor that contributes to individual differences in men's masculinity. When studies show that CAH girls and women show more masculine behaviors than non-CAH females, they suggest that androgens contribute to individual differences in women's masculinity and femininity.

Behavioral genetic studies provide another route to studying biological contributions to individual differences in masculinity and femininity. Such studies were discussed briefly in relation to sexual orientation, but behavior genetic research probes many other kinds of individual differences as well, including traits such as masculinity and femininity. By examining the similarity of traits between twins, among siblings, and among members of adoptive families, behavior geneticists try to untangle genetic and environmental causes of individual differences.

The mathematical methods of behavior genetic studies are often quite complex. The basic ideas are easy to grasp, however, if you consider simple examples, imagine that a researcher studies 100 sets of identical twins who were separated at birth and reared in unrelated families. Suppose further that the researcher measures these twins on various traits (aggressiveness, masculinity-femininity, intelligence) and determines how similar twins are on these traits. The twins are genetically identical but do not share their environments; therefore, you would probably agree that if twins are similar to one another, this similarity must be due to genetic factors.

Consider another equally extreme example: Babies adopted at birth from genetic strangers. After the adopted children grow up, we can measure various traits (aggressiveness, masculinity-femininity, intelligence) of the grown-up children and their adoptive families and we can see how similar family members are to one another. Adopted children are genetically unrelated to members of their adoptive families; therefore, if they are similar to their family members, these similarities must be due to shared environments, not to shared genes.

Actual behavior genetic studies are more complicated because twins usually are not separated at birth. Thus, most twins share both genes and environments. Furthermore, different kinds of blood relatives share different percentages of their genes by descent. Identical twins share 100% of their genes. Regular brothers and sisters as well as fraternal twins share, on average, 50% of their genes, as do parents and their biological offspring. Thus, behavior genetic studies must mathematically model varying degrees of genetic and environment similarity and investigate what degree of genetic and environmental influence best explains the observed patterns of similarity between various family members.

To make matters even more complicated but also more interesting, behavior genetic studies often distinguish between two kinds of environments: shared and unique. Shared environments are shared by all the children in a given family, and thus they should have the same effect on all the children. Examples of shared environments are the socioeconomic class of a family, the neighborhood in which the family lives, and general parenting styles that equally affect all children in the family (e.g., one mother is alcoholic and abusive, whereas another is loving and fair). A commonsense way to think of shared environmental factors is that they tend to make children in a given family similar to one another. If all the children in a given family grow up in a low socioeconomic neighborhood, then this environmental factor may depress the IQ scores of all the children in the family (i.e., make the children more similar on IQ).

In contrast, unique environmental factors afiect each child in a family differently. Each child may have different friends and teachers. Parents may treat one child differently from another. Unique environmental effects tend to make children in a given family different from one another. Imagine that Moe and Joe grew up in the same family. However, mom always loved Moe better than Joe and treated him better. Moe had caring teachers, but Joe did not. Moe hung out with a good crowd in school, but Joe joined a street gang. As a result, Moe became a model citizen, whereas Joe became a juvenile delinquent and later a violent criminal.

Behavior genetic studies typically produce estimates of the proportion of variability in a given trait (e.g., aggressiveness or masculinity femininity) that is due to genetic factors, shared environmental factors, and unique environmental factors. These proportions add up to one. For example, the behavior genetic research on intelligence suggests that among adults, 50% to 80% of variability in intelligence is due to genetic factors, and most of the rest is due to unique environmental factors (Jensen, 1998).

The percentage of variability caused by genetic factors is termed the heritability of a trait. Heritability estimates apply only to populations of people, not to individuals. A behavior genetic study may conclude that 50% of the variability of IQ scores in a given sample of people is due to genetic factors. It can never conclude, however, that 50% of Joan's IQ is due to genetic factors. Because the study of masculinity and femininity is the study of individual differences in personality, behavior genetic studies offer an important source of evidence about genetic and environmental factors that contribute to these individual differences.

Behavior genetic analyses of measures of masculinity and femininity-such as instrumental personality traits (e.g., dominance), expressive personality traits (e.g., nurturance),.sex-typed occupational preferences and interests, and omnibus M-F scales—generally indicate that these traits show significant heritability (Cleveland, Udry, & Chantala, 2001; Lippa & Hershberger, 1999; Loehlin, 1985; Mitchell, Baker, & Jacklin, 1989; Rowe, 1982). A study that examined archival data from more than 800 identical and fraternal twin pairs found that 36% of the variability in nurturance, 38% of the variability of dominance, and 53% of the variability of gender-related interests was due to genetic factors, and this was true for both males and females (Lippa & Hershberger, 1999). in addition to demonstrating potent genetic influences, this study also showed thai unique environmental factors contribute substantially to individual differences in masculinity and femininity, but common environmental factors do not, In commonsense terms, environmental influences tend to make siblings dissimilar on masculinity and femininity, not similar.

Future research may help identify the unique environmental factors that lead people to differ in their levels of masculinity and femininity. Perhaps people seek out environments that amplify their genetic differences. Maybe Bret, who is somewhat feminine, joins the drama club in high school and has a number of close female friends, whereas his brother Bart, who is more masculine, joins the football team and hangs out with the other high school jocks. Bret's innate femininity may lead him to gravitate to settings and friends that encourage his feminine interests, whereas Bart's masculinity leads him down a more macho path.

Of course, biology does not operate in a vacuum. Nature may interact with nurture. Sociologist Richard Udry's (2000) research on links between prenatal hormones and adult women's femininity provides a good example. As described earlier, Udry found that women who were exposed to high testosterone during the second trimester of their fetal development tended to be more masculine as adults than women exposed to lower levels. However, Udry also measured how much women's mothers encouraged them to be masculine or feminine as children. Interestingly, he found that women with low prenatal exposure to testosterone were more influenced by their mothers; they became more feminine when their mothers encouraged femininity, but they became more masculine when their mothers encouraged more masculine behaviors, in contrast, women with high prenatal testosterone exposure were less responsive to their mothers; they were simply more behaviorally masculine in general, regardless of whether their mothers encouraged them to be feminine or masculine. Thus some people may be more consistently influenced by biological factors, and others may be more molded by rearing.

Whatever the relative balance of nature and nurture in determining gender, biology is always part of the equation. As Cal (the man who had been reared as the girl, Cailie) observed toward the end of the novel Middlesex, "In the twentieth century, genetics brought the Ancient Greek notion of fate into our very cells." And as the 20th century merged into a new millenium, research increasingly showed that men's and women's slates were not nearly as blank as John Locke had thought.