Perhaps even more interesting than genotype-environment interaction is the concept of genotype-environment correlation, the differential exposure of individuals with different genotypes to different environments. Consider, for example, a child who has a genotype for high verbal ability . Her parents may notice this and provide her with lots of books to read, engage in intellectual discussions with her , and give her word games and crossword puzzles. Parents of children with less verbal skill, who presumably have dif ferent genotypes than those with high verbal abilities, may be less inclined to provide this stimulation. This is an example of genotype-environment correlation—whereby individuals with dif ferent genotypes (e.g., those with high versus low verbal abilities) are exposed to dif ferent environments (e.g., high versus low stimulation). In another example, parents might promote sports activities for athletically inclined children more than for less athletically inclined children.
Plomin, DeFries, and Loehlin (1977) describe three very dif ferent kinds of genotype-environment correlation: passive, reactive, and active. Passive genotype-environment correlation occurs when parents provide both genes and the environment to children, yet the children do nothing to obtain that environment. Suppose, for example, that parents who are verbally inclined pass on genes to their children that make them verbally inclined. However , because the parents are highly verbal, they buy a lot of books. Thus, there is a correlation between the children' s verbal ability and the number of books in their home, but it is passive in the sense that the child has done nothing to cause the books to be there.
In sharp contrast, the reactive genotype-environment correlation occurs when parents (or others) respond to children differently, depending on the child's genotypes. A good example is cuddlers versus noncuddlers. Some babies love to be touched— they giggle, smile, laugh, and show great pleasure when they are handled. Other babies are more aloof and simply do not like to be touched very much. Imagine that a mother starts out touching and hugging each of her two children a lot. One child loves it; the other hates it. Over the course of several months, the mother reacts by continuing to hug the cuddler but cuts down on hugging the noncuddler. This example illustrates the reactive genotype-environment correlation, which is achieved because people react to children dif ferently, based in part on the children' s heritable dispositions, such as a liking for being cuddled.
Active genotype-envir onment corr elation occurs when a person with a particular genotype creates or seeks out a particular environment. High sensation seekers, for example, expose themselves to risky environments—skydiving, motorcycle jumping, and drug taking. Highly intellectual individuals are likely to attend lectures, read books, and engage others in verbal discourse. This active creation and selection of environments has also been called "niche picking" (Scarr & McCartney, 1983). Active genotype-environment correlation highlights the fact that we are not passive recipients of our environments; we mold, create, and select the environments we subsequently inhabit, and some of these actions are correlated with our genotypes.
These genotype-environment correlations can be positive or negative. That is, the environment can encourage the expression of the disposition, or it can discourage its expression. For example, parents of highly active children may try to get them to sit still and calm down, and parents of less active children may try to get them to perk up and be more lively , in which case there is a negative genotype-environment correlation because the parents' behavior opposes the children's traits (Buss, 1981). Another example of negative genotype-environment correlation occurs when people who are too dominant elicit negative reactions from others, who try to "cut them down" (Cattell, 1973). The key point is that environments can go against a person' s genotype, resulting in a negative genotype-environment correlation, or they can facilitate the person' s genotype, creating a positive genotype-environment correlation.
A recent study of 180 twins reared apart points to an intriguing potential example of genotype-environment correlation (Krueger , Markon, & Bouchard, 2003). The study assessed personality traits through the Multidimensional Personality Questionnaire (MPQ), which identifies three major factors of personality: Positive Emotional ity (happy , content), Negative Emotionality (anxious, tense), and Constraint (controlled, conscientious). Then they evaluated each individual' s perceptions of the family environments in which they were raised, which yielded two main factors: Family Cohesion (e.g., parental warmth, absence of family conflict) and Family Statu (e.g., parents provided intellectual and cultural stimulation, active recreational activities, and financial resources). The intriguing results were that the correlations between personality and perceptions of family environment were genetically mediated. In other words, the perceived environment in which the individuals were raised was lar gely due to heritable personality traits. Specificall , experiencing a cohesive family upbringing was explained by genetic influence on the two personality traits of Constraint and
lack of Negative Emotionality . In contrast, recalling a family environment high in cultural, intellectual, and economic status was explained by the heritable personality trait of Positive Emotionality .
These results may be subject to several interpretations. One interpretation is that personality affects the subjective manner in which people remember their early environments. Perhaps calm, controlled individuals are more likely to for get about real family conflict that was present during their childhood, and so may simpl recall greater family cohesion than actually existed. An alternative interpretation is in terms of genotype-environment correlation: Individuals with calm, controlled personalities (high Constraint, low Negative Emotionality) may actually promote cohesion among family members—in essence, creating a family environment that further fosters their calm, controlled personality . Future studies of personality , parenting, and perceived family environments of fer the promise of unraveling the subtle and complex ways in which genes interact and correlate with environments (Spinath & O'Connor, 2003).
The concepts of genotype-environment interaction and correlation are intriguing in providing a more complex picture of human personality functioning. It is clear from behavioral genetic studies that both heredity and shared and nonshared environments influence personalit . It will be exciting to follow these lines of research over the next decade as they document the precise nature of these interactions and correlations.
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