With all of the findings on the moderate heritability of so many personality charac teristics, it is important not to lose sight of one important fact: the same studies that suggest moderate heritability also provide the best evidence for the importance of environmental influences. If many personality characteristics show heritabilities in th range of 30 to 50 percent, this means that the same characteristics show a substantial degree of environmentality—as much as 50 to 70 percent. This conclusion must be tempered, however, by the fact that all measures are flawed, containing errors of mea surement; some of the dif ferences in personality might be attributable to neither environmental nor genetic dif ferences but, rather, to error of measurement. Nonetheless, because behavioral genetic evidence points to the importance of environmental influ ences on personality , behavioral geneticists have turned increasingly to the issue of how their methods can be used to provide insights into the nature of environmental influences
One critical distinction behavioral geneticists make is between shared and nonshared environmental influences Consider siblings—brothers and sisters in the same family. Some features of their environment are shared—the number of books in the home, the presence or absence of a TV, DVD player, or computer, the quality and quantity of food in the home, the parents' values and attitudes, and the schools, church, synagogue, or mosque the parents send the children to. All of these are features of the shared environment. On the other hand, the same brothers and sisters do not share all features of their environment. Some children might get special treatment from their parents. They might be labeled dif ferently by their parents. They might have different groups of friends. They might occupy dif ferent rooms in the house. One might go to summer camp, whereas the others stay home each summer . All of these features are called nonshared because they are experienced dif ferently by different siblings.
Make a list of five shared environmental influences you have in common with your siblings (or, if you are an only child, what things might be shared environmental influences if you had siblings?). Then list five nonshared environmental influences. Which had the strongest influence on your personality, attitudes, or behavior?
We know that the environment exerts a major influence on personality—i accounts for a substantial share of the variance. But which environment matters most—the shared or the nonshared environment? Some behavioral genetic designs allow us to figure out whether the environmental e fects come more from shared or from nonshared sources. The details of how this is done are too technical to examine in this book, but, if you are interested, you can check out the fascinating article by Plomin and Daniels (1987) for more details.
The bottom line is this: for most personality variables, the shared environment has either little or no discernible impact. Adoption studies, for example, show that the average correlation for personality variables between adopted siblings who share much of their environment, but who share no genes, is only .05. This suggests that, even though these siblings are growing up together—with the same parents, same schools, same religious training, and so on—whatever is happening in their shared environment (e.g., parenting, rearing practices, values education) is not causing them to be similar in personality .
Instead, most environmental causes appear to stem from the aspects of the environment that siblings experience dif ferently. Thus, it's not the number of books in the home. It's not parental values or parental attitudes toward child rearing. In fact, it' s not what most psychologists have long believed it is. Rather , the critical environmental influences on personality appear to lie in the unique experiences of individual children
These findings should not be surprising. Identical twins, and even nontwin sib lings who grow up together , may work to create their own identities, cultivate their own skills, and for ge their unique paths in life. In the case of identical twins reared together, people may have a vested interest in telling them apart and, so, create an environment that emphasizes the differences between them. The key point is that environments matter tremendously in the development of personality , but not the environmental features that siblings share. Their unique environments and experiences, instead, appear to be critical for the development of personality .
Which unique experiences are important? Well, here we run into a brick wall. The discovery of the importance of the nonshared environment is recent, coming to the attention of the scientific community only within the past few years. Most theo ries of socialization over the decades have focused exclusively on the shared environment, such as parental attitudes toward child rearing. Thus, it is only recently that psychologists have begun to study nonshared environments.
There are two possibilities of what they will find. One possibility is a majo breakthrough—a discovery of a critically important environmental variable that has been overlooked by psychologists who for years focused only on the shared environment. The other possibility is less satisfying. It is conceivable that there are so many environmental variables that exert an impact on personality that each one alone might account only for a tiny fraction of the variance (W illerman, 1979). If this is the case, then we are stuck with the discovery of many small ef fects.
Does this mean that the shared environment accounts for nothing? Have psychologists been entirely misguided in their thinking by their focus on shared ef fects? The answer is no. In some areas, behavioral genetic studies have revealed tremendously important shared environmental influences: attitudes, religious beliefs, politi cal orientations, health behaviors, and to some degree verbal intelligence (Segal, 1999). As an example, adoptive siblings reared together but genetically unrelated correlated .41 (girls) and .46 (boys) in their patterns of smoking and drinking (W illerman, 1979). Thus, although smoking and drinking have a substantial genetic component, there is also a lar ge shared environmental component.
Another recent study found that shared environments accounted for several personality clusters in the "adjustment" domain (Loehlin, Neiderhiser , & Reiss, 2003). These include antisocial behavior (e.g., showing behavior problems and breaking rules), depressive symptoms (e.g., moody , withdrawn), and autonomous functioning
(e.g., being able to care for self in basic needs and recreational activities). And a study of adult twins using observational measurement—trait ratings of videotaped behaviors— suggests that shared environment might be more important in explaining Big Five personality traits than is typically revealed by studies using self-report (Borkenau, Reimann, Angleitner, & Spinath, 2001). If this study is replicated by future research, it may have the far -reaching consequence of challenging the now-conventional wisdom that shared environments have little ef fect on personality traits.
Discuss what you think might represent shared environmental influences that contribute to the tendency to smoke. That is, what in the environment might have influenced most people who smoke to start and maintain their smoking habit?
In summary, environments shared by siblings are important in some domains. But, for many personality traits, such as extraversion and neuroticism, shared environments do not seem to matter . Instead, it is the unique environment experienced by each sibling that carries the causal weight.
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