To understand the primary goals of the field of behavioral genetics, let s look at a concrete example—individual differences in height. Some individuals are tall, such as basketball player Shaquille O'Neal (over 7 feet). Other individuals are short, such as actor Danny DeV ito (around 5 feet). Geneticists focus on the key question, "What causes some individuals to be tall and others to be short?" In other words, what are the causes of individual dif ferences in height?
In principle, there can be a variety of causes of individual height dif ferences. Differences in diet while growing up, for example, can cause dif ferences in height among people. Genetic dif ferences can also account for some of the dif ferences in height. One of the central goals of genetic research is to determine the percentage of an individual difference that can be attributed to genetic dif ferences and the percentage that is due to environmental dif ferences.
In the case of height, both environmental and genetic factors are important. Clearly, children tend to resemble their parents in height—generally , tall parents have taller than average children and short parents have shorter than average children. And u
In determining height, genetics accounts for 90 percent of the variation, while environmental factors, such as diet, account for 10 percent of the variation. The actor Danny DeVito (left) is about 2 feet shorter than basketball player Shaquille O'Neal (right).
genetic research has confirmed that roughly 90 percent of the individual di ferences in height are indeed due to genetic dif ferences. The environment, which contributes 10 percent to individual dif ferences in height, is far from trivial. In the United States, average adult height has increased in the entire population by roughly 2 inches over the past century, most likely due to increases in the nutritional value of the food eaten by U.S. citizens. This example brings home an important lesson: even though some observed differences between people can be due to genetic dif ferences, this does not mean that the environment plays no role in modifying the trait.
Can you think of some human characteristics that you consider mostly under genetic influence? Consider, for example, individual differences in eye color. Can you think of other characteristics that are not very much influenced by genetic factors? Consider, for example, individual differences in eating with forks versus eating with chopsticks. How might you go about proving that some individual differences are, or are not, influenced by genetic differences?
The methods used by behavioral geneticists, which we will examine in this chapter, can be applied to any individual dif ference variable. They can be used to identify the causes of individual dif ferences in height and weight, dif ferences in intelligence, differences in personality traits, and even dif ferences in attitudes, such as liberalism or conservatism, and preferences for particular styles of art. The methods have been applied to all of these phenomena.
However, behavioral geneticists are typically not content simply with figurin out the percentage of variance due to genetic and environmental causes. Percentage of variance refers to the fact that individuals vary , or are dif ferent from each other , and this variability can be partitioned into percentages that are due to dif ferent causes. Behavioral geneticists also are interested in determining the ways in which genes and the environment interact and correlate with each other . And they are interested in fig uring out precisely where in the environment the ef fects are taking place—in parental socialization practices, for example, or in the teachers to which children are exposed. We will turn to these more complex issues toward the end of this chapter . But, first we must examine the fundamentals of behavioral genetics: What is heritability, and what methods do geneticists use to get their answers?
What Is Heritability?
Heritability is a statistic that refers to the proportion of observed variance in a group of individuals that can be accounted for by genetic variance (Plomin, DeFries, McClearn, & McGuf fin, 2001). It describes the degree to which genetic di ferences between individuals cause dif ferences in an observed property , such as height, extraversion, or sensation seeking. Heritability may be one of the most frequently misunderstood concepts in psychology . If precisely defined, howeve , it provides useful information in identifying the genetic and environmental determinants of personality .
Heritability has a formal definition: the proportion of phenotypic variance that is attributable to genotypic variance . Phenotypic variance refers to observed individual dif ferences, such as in height, weight, or personality . Genotypic variance refers to individual dif ferences in the total collection of genes possessed by each person. Thus, a heritability of .50 means that 50 percent of the observed phenotypic variation is attributable to genotypic variation. A heritability of .20 means that only 20 percent of the phenotypic variation is attributable to genotypic variation. In these examples, the environmental component is simply the proportion of phenotypic variance that is not attributable to genetic variance. Thus, a heritability of .50 means that the environmental component is .50. A heritability of .20 means that the environmental component is .80. These examples illustrate the simplest cases and assume that there is no correlation or interaction between genetic and environmental factors.
The environmental contribution is defined in a similar wa . Thus, the percentage of observed variance in a group of individuals that can be attributed to environmental (nongenetic) dif ferences is called environmentality. Generally speaking, the larger the heritability, the smaller the environmentality . And vice versa—the smaller the heritability, the larger the environmentality.
Discuss the meaning of the following statement: "All normally developing humans have language, but some people speak Chinese, others French, and others English." To what degree is variability in the language spoken due to variability in genes or variability in the environment in which one is raised?
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