Perhaps the most direct way to study the role of genetics in personality is to selectively breed animals for the trait of interest. In this approach the scientist chooses the animals that most strongly express the personality characteristics of interest and breeds these animals with each other. If the selective breeding creates offspring with even stronger traits, then we can assume that the trait has genetic origins. In this manner, scientists have studied the role of genetics in how worms respond to stimuli, how fish develop courtship rituals, how rats differ in play, and how pigs differ in their responses to stress.
Although selective breeding studies can be informative, they are clearly not useful for studying humans. For this psychologists rely onbehavioral genetics—a variety of research techniques that scientists use to learn about the genetic and environmental influences on human behavior by comparing the traits of biologically and nonbiologically related family members (Baker, 2010). 1Behavioral genetics is based on the results of familystudies, twinstudies, and adoptivestudies.
A family study starts with oneperson who has a trait of interest—for instance, a developmental disorder such as autism—and examines theindividual’s familytreeto determinetheextent to which other members of thefamilyalso havethetrait. The presence of the trait in first-degree relatives (parents, siblings, and children) is compared to the prevalence of the trait in second- degree relatives (aunts, uncles, grandchildren, grandparents, and nephews or nieces) and in more distant family members. The scientists then analyze the patterns of the trait in the family members to see the extent to which it is shared by closer and more distant relatives.
Although family studies can reveal whether a trait runs in a family, it cannot explain why. In a twin study, researchers study the personality characteristics of twins. Twin studies rely on the fact that identical (or monozygotic) twins have essentially the same set of genes, while fraternal (or dizygotic) twins have, on average, a half-identical set. The idea is that if the twins are raised in the same household, then the twins will be influenced by their environments to an equal degree, and this influence will be pretty much equal for identical and fraternal twins. In other words, if environmental factors are the same, then the only factor that can make identical twins more similar than fraternal twins is their greater genetic similarity.
In a twin study, the data from many pairs of twins are collected and the rates of similarity for identical and fraternal pairs are compared. A correlation coefficient is calculated that assesses the extent to which the trait for one twin is associated with the trait in the other twin. Twin studies divide the influence of nature and nurture into three parts:
Heritability (i.e., genetic influence)is indicated when the correlation coefficient for identical twins exceeds that for fraternal twins, indicating that shared DNA is an important determinant of personality.
Shared environment determinants are indicated when the correlation coefficients for identical and fraternal twins are greater than zero and also very similar. These correlations indicate that both twins are having experiences in the family that make them alike.
Nonshared environment is indicated when identical twins do not have similar traits. These influences refer to experiences that are not accounted for either by heritability or by shared environmental factors. Nonshared environmental factors are the experiences that make individuals within the same family lessalike. If a parent treats one c hild more affectionately than another, and as a consequence this child ends up with higher self-esteem, the parenting in this case is a nonshared environmental factor.
In the typical twin study, all three sources of influence are operating simultaneously, and it is possible to determine the relative importance of each type.
An adoption study compares biologically related people, including twins, who have been reared either separately or apart. Evidence for genetic influence on a trait is found when children who have been adopted show traits that are more similar to those of their biological parents than to those of their adoptive parents. Evidence for environmental influence is found when the adoptee is more like his or her adoptive parents than the biological parents.
The results of family, twin, and adoption studies are combined to get a better idea of the influence of genetics and environment on traits of interest. Table 11.6 presents data on the correlations and heritability estimates for a variety of traits based on the results of behavioral genetics studies (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). 2
|Correlation between children raised together||Correlation between children raised apart||Estimated percent of total due to|
|Identical twins||Fraternal twins||Identical twins||Fraternal twins||Heritability (%)||Shared environment (%)||Nonshared environment (%)|
|Age of puberty||45||5||50|
|General cognitive ability||56||0||44|
|Likelihood of divorce||0.52||0.22|
|Big Five dimensions||40–50|
|This table presents some of the observed correlations and heritability estimates for various characteristics.|
If you look in the second column of Table 11.6, you will see the observed correlations for the traits between identical twins who have been raised together in the same house by the same parents. This column represents the pure effects of genetics, in the sense that environmental differences have been controlled to be a small as possible. You can see that these correlations are higher for some traits than for others. Fingerprint patterns are very highly determined by our genetics (r= .96), whereas the Big Five trait dimensions have a heritability of 40–50%.
You can also see from the table that, overall, there is more influence of nature than of parents. Identical twins, even when they are raised in separate households by different parents (column 4), turn out to be quite similar in personality, and are more similar than fraternal twins who are raised in separate households (column 5). These results show that genetics has a strong influence on personality, and helps explain why Elyse a nd Paula were so similar when they finally met.
Despite the overall role of genetics, you can see in Table 11.6 that the correlations between identical twins (column 2) and heritability estimates for most traits (column 6) are substantially less than 1.00, showing that the environment also plays an important role in personality (Turkheimer & Waldron, 2000). 3 For instance, for sexual orientation the estimates of heritability vary from 18% to 39% of the total across studies, suggesting that 61% to 82% of the total influence is due to environment.
You might at first think that parents would have a strong influence on the personalities of their children, but this would be incorrect. As you can see by looking in column 7 of Table 11.6, research finds that the influence of shared environment (i.e., the effects of parents or other caretakers) plays little or no role in adult personality (Harris, 2006). 4 Shared environment does influence the personality and behavior of young children, but this influence decreases rapidly as the c hild grows older. By the time we reach adulthood, the impact of shared environment on our personalities is weak at best (Roberts & DelVecchio, 2000). 5 What this means is that, although parents must provide a nourishing and stimulating environment for children, no matter how hard they try they are not likely to be able to turn their children into geniuses or into professional athletes, nor will they be able to turn them into criminals.
If parents are not providing the environmental influences on the child, then what is? The last column in Table 11.6, the influence of nonshared environment, represents whatever is “left over” after removing the effects of genetics and parents. You can see that these factors—the largely unknown things that happen to us that make us different from other people—often have the largest influence on personality.