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Heritability often comes up in discussions of the nature-nuture question. Heritability is often misinterpreted as describing how much heredity contributes to some trait that an individual shows. This is WRONG! A more accurate, but simplified, definition is this: Heritability is the proportion of this total variation between individuals in a given population due to genetic variation. This number can range from 0 (no genetic contribution) to 1 (all differences on a trait reflect genetic variation).
Two important points follow from this definition:
1. The number does NOT apply to individuals -- only to variations within a group (or population).
2. The number is NOT fixed. Differences among groups in range of genetic variation and/or environmental variation will result in different estimates of heritability.
Heritability reflects the fact that all individuals in any species of living things differ in many ways among each other. The variation (~differences) among individuals within a species depends on both genetic and environmental differences. For example, people differ in height. How much of this variation among people is due to genetic differences, how much is due to environmental differences, and how much is due to their interaction? Both genetic and environmental factors contribute. Taller parents have on the average taller children than do shorter parents, and identical twins are almost identically tall. These and other data show a genetic role in height differences. However, Japanese-Americans are on the average taller and heavier than their second cousins who grew up in Japan. This is one of kind of data that indicates the role of environment.
Heritability sounds simple, but it can act in unexpected ways. For example, if everyone in a population has the same allele for a trait and shows little variation (differences) on that trait, then the heritability for that trait is zero. It is zero because that trait has no genetic variation. One example is hair color among Eskimos (N. Carlson & W Buskirt, 1997). The whole Eskimo population appears to have the same alleles for hair color, so for Eskimos, the heritability for hair color is 0.00, even though the color is under strong genetic control! It's 0.00 because there is no genetic variation for hair color.
The calculations for heritability were originally developed for experiments on plant and animal breeding, but they have also been applied to human genetic processes. The application to behavioral processes has been controversial. Some scientists believe that the calculations require assumptions that human behavior genetic data do not meet. Therefore, these scientists believe that calculations of heritability for psychological processes like intelligence and personality factors are invalid.
Behavior geneticists disagree. They calculate heritability, using statistical models of twin and adoption data, for processes like intelligence, extraversion (outgoing personality), mental abnormality, etc. For example, heritability of extraversion is estimated at about 0.35, based on data from several studies done in different parts of the world (Loehlin, 1992).
The frequent misinterpretation of heritability reflects the traditional nature versus nurture question, which, as mentioned above, is the wrong question. A trait cannot be divided into environmentally and genetically controlled parts. To repeat once more: Everything about an animal or a plant depends both on heredity and on the environment in which its heredity is expressed. Every gene must express itself in an environment, and all environments must act on the genotype an individual gets.
We call some traits "genetically controlled." These traits are little affected by the range of environments that a developing individual normally encounters. For example, the human body is programmed to develop arms and hands with five fingers. But this genetic program can encounter a very unusual environment, which disrupts expression of this program. For example, the human embryo (very early stage of development after conception) develops very abnormal hands and arms if it is exposed to the medication thalidomide early in development.
Factors that we call "environmentally controlled" depend much more on the environment in which the individual develops. For example, the language a person speaks depends on the speech environment in which s/he develops. But all humans learn some language, even under many language-deprived conditions. This fact implies that the human brain is genetically programmed to develop language with a system specialized for language. Many linguists (people who study language) claim that all languages have basic universal features.