The history of behavioral genetic research has taken some fascinating twists and turns, which are worth noting (see Plomin et al., 1990, for an excellent summary of this history). During the past century in the United States, behavioral genetic research received what can be phrased as a "frosty reception." Findings that some personality traits were moderately heritable seemed to violate the dominant paradigm, which was environ-mentalism (and, especially, behaviorism). The prevailing environmentalist view was that personality was determined by socialization practices, such as parenting style. Furthermore, people worried about the potential misuse of findings eme ging from behavioral genetics. Images of Nazi Germany sprang to mind, with the evil notions of a master race. Of course, there is the notion of ethnic cleansing, which has strong genetic overtones.
A large part of the controversy over genetic research on personality has centered around studies of intelligence, which has often been considered to be a personality variable. Many people have worried that findings from these studies will b misused to label some people intrinsically superior or inferior to others (e.g., see Herrnstein & Murray, 1994). Others worry that findings will be misused to give som people preferential treatment in education or job placement. Still others are concerned that standard tests of intelligence fail to capture many of the multiple facets of intelligence, such as social intelligence, emotional intelligence, and creativity . All of these are legitimate concerns, and they suggest that the findings from the field of beha ioral genetics must be viewed with caution and interpreted responsibly , in terms of the larger picture of human nature and society .
In the past decade, attitudes have shifted somewhat, and the field of psychol ogy now considers the findings from behavioral genetics as fairly mainstream. Behav ioral genetic studies tend not to generate the intense controversy that they did in prior decades. One recent exception to this are the studies on the heritability of sexual orientation, which generated some media controversy . For example, if homosexuality is more environmental and learned than was previously thought, then some groups have suggested that homosexuality could be unlearned, or "cured."
The links between science and politics, between knowledge and values, are complex, but they need to be confronted. Because scientific research can be misuse for political goals, scientists bear a major responsibility for presenting findings care fully and accurately. Some argue that science and values cannot be separated and that even science itself is a political tool used to oppress certain people. There may be no subdiscipline for which these complex issues of the mingling of science and values is more relevant than the field of behavioral genetics
Science can be separated from values. Science is a set of methods for discovering what exists. Values are notions of what people want to exist—to be desired or sought after. Although scientists clearly can be biased by their values, the virtue of the scientific method is that it is self-correcting. The methods are public, so other scientists can check the findings, discover errors in procedure, and, hence, over time cor rect any biases that creep in. This does not imply , of course, that scientists are unbiased. Indeed, the history of science is filled with cases in which values influenc the nature of the questions posed and the acceptance or rejection of particular find ings or theories. Nonetheless, the scientific method provides a method for correctin such biases in the long run.
SUMMARY AND EVALUATION_
The behavioral genetics of personality has a fascinating history in the twentieth century. Early on, when behavioral genetic methods were being developed, the field o psychology was dominated by the behaviorist paradigm. In this context, findings fro behavioral genetic research were not warmly received. Furthermore, social scientists worried that findings from behavioral genetic research might be misused for ideolog ical purposes.
Over the past two decades, the empirical evidence on heritability has become stronger and stronger, in part because of the conver gence of evidence across behavioral genetic methods. There are four major behavioral genetic methods: selective breeding studies, family studies, twin studies, and adoption studies. Selective breeding studies cannot be ethically conducted on humans. Family studies are problematic because the genetic and environmental factors are often confounded. Twin studies have potential problems, such as violations of the equal environments assumption (the assumption that identical twins are not treated any more alike than fraternal twins) and the assumption of representativeness (the notion that twins are just like nontwins). Adoption studies also have potential problems, such as the non-random placement of adopted-away children in particular families and, like twin studies, the assumption of representativeness (the notion that adopted children are like nonadopted children in all key respects). Empirical tests of these assumptions suggest that they are not violated much or are violated in ways that do not seem to make much dif ference. However, the most compelling evidence on the heritability of personality comes from looking across methods that do not share methodological problems. Thus, if the findings from twin studies and adoption studies converge on the same result, then we can have more confidence in the results than we can whe just a single method is used.
The study of lar ge samples of twins reared together , the study of smaller samples of identical twins reared apart, and sound adoption studies have added greatly to the credibility of behavioral genetic research. The empirical findings clearly show tha personality variables, such as extraversion and neuroticism, as well as the other dimensions of the Big Five, have moderate heritability . Perhaps even more striking are the findings that drinking, smoking, attitudes, occupational preferences, and eve sexual orientation appear to be moderately heritable. Equally important, however , is the finding that the same studies provide the best evidence for the importance of envi ronmental influences. Overall, personality characteristics are 30 to 50 percent herita ble and 50 to 70 percent environmental.
Perhaps most interesting, the environmental causes appear to be mostly of the nonshared variety—that is, the dif ferent experiences that siblings have even though they are in the same family . This finding is so startling because nearly all theorie of environmental influence—such as those that posit the importance of parental val ues and child-rearing styles—have been of the shared variety . Thus, behavioral genetic research may have provided one of the most important insights into the nature of nurture—the location of the most important environmental influences on person ality. The next decade of personality research should witness progress in identifying the precise locations of these nonshared environmental influences. Separating perceived environments from objective environments will be an important part of this research program.
In interpreting the research findings, it is important to keep in mind the mean ing of heritability and the meaning of environmentality . Heritability is the proportion of observed individual dif ferences that are caused by genetic dif ferences in a particular population or sample. It does not pertain to an individual, since genetic and environmental influences are inextricably interwoven at the individual level and cannot b separated. Heritability does not mean that the environment is powerless to alter the individual differences. And heritability is not a fixed statistic—it can be low in on group and high in another , low at one time and high at another . Environmentality is the proportion of observed individual dif ferences that is caused by environmental differences. Like heritability, environmentality is not a fixed statistic. It, too, can chang over time and across situations. The discovery of a powerful environmental intervention, for example, could, in principle, dramatically increase environmentality while lowering heritability. The key point is that neither heritability nor environmentality is fixed in space and time
In addition to providing estimates of heritability and environmentality , some behavioral genetic research examines the interactions and correlations between genetic and environmental variables. There are three major types of genotype-environment correlations—passive, reactive, and active. Passive genotype-environment correlation occurs when parents provide both genes and environment to their children in ways that just happen to be correlated—for example, parents who pass on genes for verbal ability and stock their houses with a lot of books. Books and verbal ability become correlated, but in a passive way , since the children did not have to do anything for the correlation to occur. Reactive genotype-environment correlation occurs when parents, teachers, and others respond dif ferently to some children than to others. Parents generally tickle and coo at smiley babies more than at nonsmiley babies, creating a correlation between genotypes for smiling and a cuddly social environment. The correlation occurs because parents react to babies dif ferently. Active genotype-environment correlation occurs when individuals with certain genotypes seek out environments nonrandomly. Extraverted individuals, for example, might throw a lot of parties, thus surrounding themselves with a dif ferent social environment than that of the more reclusive introverts. The correlation occurs because individuals actively create it.
The more complex and interesting behavioral genetic concepts such as genotype-environment correlation have received relatively little research attention. A recent possible exception is the fascinating finding that individuals low on Negative Emo tionality and high on Constraint recall their early family environment as being extremely cohesive. One interpretation is in terms of genotype-environment correlation: Calm, nonneurotic individuals may actually promote calmness and cohesion in their family environment, thus creating an upbringing that further fosters their calm, controlled personality . Now that some of the basic estimates of heritability and environmentality have been established, however , the next wave of research may reveal the more complex nature of the causes of individual dif ferences in personality.
Molecular genetics represents the most recent development in the realm of personality psychology. The research techniques attempt to establish an association between specific genes and scores on personality traits. Initial findings of a li between the D4DR gene and novelty seeking, however , have not been successfully replicated. More recent work has focused on possible genes underlying neuroticism— specificall , genes involved in the serotonin system.
Genome 174 Genetic Junk 174 Eugenics 175 Percentage of Variance 177 Heritability 177 Phenotypic Variance 178 Genotypic Variance 178 Environmentality 178 Nature-Nurture Debate 179 Selective Breeding 180 Family Studies 181
Twin Studies 182
Monozygotic (MZ) Twins 182
Dizygotic (DZ) Twins 182
Equal Environments Assumption 183
Adoption Studies 184
Selective Placement 185
Gender Identity Disorder (GID) 191
Shared Environmental Influence 193
Influence 193 Genotype-Environment Interaction 195
Correlation 196 Passive Genotype-Environment
Correlation 196 Reactive Genotype-Environment
Correlation 196 Active Genotype-Environment
Correlation 197 Molecular Genetics 198 D4DR Gene 198 Environmentalist View 199
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