I recently read a statement that goes like this: heritability is a population construct. Isn't heritability dependent on genetic differences? Can someone provide an example of what it means to be a population construct? Thanks
Heritability is the proportion of differences in some observable trait (the phenotype; e.g., intelligence, height, love for pizza) in a population that corresponds to differences in the genotype.
Heritability is estimated by comparing people who differ in terms of their genetic similarity and the non-genetic influences they are exposed to. For example, we know that monozygotic twins share 100% of their genes, whereas dizygotic twins share only 50% of their genes on average. If we compare twins that were raised in different homes (adopted twins) and monozygotic twins are more similar to each other than dizygotic twins we conclude that this difference overlaps with genetic differences, as expressed in a high heritability index.
Importantly, heritability does not describe the extent to which a person's individual characteristics were formed by genes or the environment, which is a frequent misconception. Heritability only describes the variance of a trait within a given population that corresponds to genetic differences in that population.
As described in Griffiths et al. (2000):
It must be stressed that this measure of “genetic influence” tells us what part of the population’s variation in phenotype can be assigned to variation in genotype. It does not tell us what parts of an individual’s phenotype can be ascribed to its heredity and to its environment. This latter distinction is not a reasonable one. An individual’s phenotype is a consequence of the interaction between its genes and its sequence of environments. It clearly would be silly to say that you owe 60 inches of your height to genes and 10 inches to environment. All measures of the “importance” of genes are framed in terms of the proportion of variance ascribable to their variation.
How this restricts the interpretation of heritability becomes clearer if we imagine cases in which there is no genetic variation (everybody has the same gene) or there is no variation in the environment (everybody is raised in the same way). In the first case, every variation would correspond to environmental differences, in the second case every variation would correspond to genetic differences.
A nice example is described here:
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.
Furthermore, it is important to point out that there can be considerable overlap between genes and environment. People with certain genes can search specific environments that others with different genes would not. These environments can then have a high impact on them. Nevertheless, because of the way in which heritability is defined, these environmental differences are ascribed to genetic differences. Furthermore, they depend on the possibility of seeking certain environments in a society. An example is described by Neisser and colleagues (1996):
A high heritability does not mean that the environment has no impact on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general psychometric intelligence) although every word in an individual’s vocabulary is learned. In a society in which plenty of words are available in everyone’s environment-especially for individuals who are motivated to seek them out-the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.
For these reasons, heritability is a population construct, it hinges on the genetic and environmental variability within the population it describes.
Griffiths, A. J., et al. (2000). An Introduction to Genetic Analysis (7th ed.). New York: W. H. Freeman. http://www.ncbi.nlm.nih.gov/books/NBK21766/
Neisser, U., Boodoo, G., Bouchard Jr., T. J., Wade, A., Brody, N., Ceci, S. J., Halpern, D. F., et al. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51, 77–101. doi:10.1037/0003-066X.51.2.77