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- Main article: Heritability
It is important to note that the term heritability refers only to the degree of genetic variation between people on a trait. It does not refer to the degree to which a trait of a particular individual is due to environmental or genetic factors. The traits of an individual are always a complex interweaving of both. For an individual, even strongly genetically influenced, or "obligate" traits, such as eye color, assume the inputs of a typical environment during ontogenetic development (e.g., certain ranges of temperatures, oxygen levels, etc.).
In contrast, the "heritability index" statistically quantifies the extent to which variation between individuals on a trait is due to variation in the genes those individuals carry. In animals where breeding and environments can be controlled experimentally, heritability can be determined relatively easily. Such experiments would be unethical for human research. This problem can be overcome by finding existing populations of humans that reflect the experimental setting the researcher wishes to create.
One way to determine the contribution of genes and environment to a trait is to study twins. In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. In another kind of twin study, identical twins reared together (who share family environment and genes) are compared to fraternal twins reared together (who also share family environment but only share half their genes). Another condition that permits the disassociation of genes and environment is adoption. In one kind of adoption study, biological siblings reared together (who share the same family environment and half their genes) are compared to adoptive siblings (who share their family environment but none of their genes).
In many cases, it has been found that genes make a substantial contribution, including psychological traits such as intelligence and personality. Yet heritability may differ in other circumstances, for instance environmental deprivation. Examples of low, medium, and high heritability traits include:
|Low heritability||Medium heritability||High heritability|
|Specific language||Weight||Blood type|
|Specific religion||Religiosity||Eye color|
Twin and adoption studies have their methodological limits. For example, both are limited to the range of environments and genes which they sample. Almost all of these studies are conducted in Western, first-world countries, and therefore cannot be extrapolated globally to include poorer, non-western populations. Additionally, both types of studies depend on particular assumptions, such as the equal environments assumption in the case of twin studies, and the lack of pre-adoptive effects in the case of adoption studies.