The concept of heritability plays a central role in the psychology of individual differences. Heritability has two definitions. The first is a statistical definition, and it defines heritability as the proportion of phenotypic variance attributable to genetic variance. The second definition is more common "sensical". It defines heritability as the extent to which genetic individual differences contribute to individual differences in observed behavior (or phenotypic individual differences). You should memorize both of these definitions.
Because heritability is a proportion, its numerical value will range from 0.0 (genes do not contribute at all to phenotypic individual differences) to 1.0 (genes are the only reason for individual differences). For human behavior, almost all estimates of heritability are in the moderate range of .30 to .60.
The quantity (1.0 - heritability) gives the environmentability of the trait. Environmentability has an analogous interpretation to heritability. It is the proportion of phenotypic variance attributable to environmental variance or the extent to which individual differences in the environment contribute to individual differences in behavior. If the heritability of most human behaviors is in the range of .30 to .60, then the environmentability of most human behaviors will be in the range of .40 to .70.
There are five important attributes about estimates of heritability and environmentability. They are:
- Heritability and environmentability are abstract concepts. No matter what the numbers are, heritability estimates tell us nothing about the specific genes that contribute to a trait. Similarly, a numerical estimate of environmentability provides no information about the important environmental variables that influence a behavior.
- Heritability and environmentability are population concepts. They tell us nothing about an individual. A heritability of .40 informs us that, on average, about 40% of the individual differences that we observe in, say, shyness may in some way be attributable to genetic individual difference. It does NOT mean that 40% of any person's shyness is due to his/her genes and the other 60% is due to his/her environment.
- Heritability depends on the range of typical environments in the population that is studied. If the environment of the population is fairly uniform, then heritability may be high, but if the range of environmental differences is very large, then heritability may be low. In different words, if everyone is treated the same environmentally, then any differences that we observe will largely be due to genes; heritability will be large in this case. However, if the environment treats people very differently, then heritability may be small.
- Environmentability depends on the range of genotypes in the population studied. This is the converse of the point made above. However, it probably does not apply strongly to human behavior as it does to the behavior of specially bred animals. Few--if any--human populations are as genetically homogeneous as breeds of dogs, sheep, etc.
- Heritability is no cause for therapeutic nihilism. Because heritability depends on the range of typical environments in the population studied, it tells us little about the extreme environmental interventions utilized in some therapies.
Below are the links to the heritability learning exercises. Click on the first one to get started.
Heritability I: Scatterplots.
Heritability II: Environmental variation around a genotypic value.
Heritability III: Culture and heritability.
Heritability IV: Between-group Heritability.