The different ways that researchers and others invoke hereditary versus environmental influences are often subsumed under the label nature versus nurture. Let me, in contrast, distinguish several disjunct areas of nature-nurture science:
- Researchers can try to compare how much variation in a trait is associated with differences among variety, location, variety-location combination, and residual contributions (i.e., genotypic, environmental, genotype-environment interaction, and residual variance).
- Researchers can try to compare how much variation is associated with differences in measured genetic factors, environmental factors, gene-environment interaction, and a residual component.
- Either through type 1 or 2 analyses, researchers can compare the variation within groups (e.g., among Euro-Americans and among African-Americans) to the difference between the averages for the groups.
- Through investigations that might extend one or both kinds of analysis of observational data, researchers can piece together a picture of the processes of development of a trait and, on that basis, speak to the fixity versus flexibility of traits.
- The evolutionary basis for human traits.
- and more…
In the long history of nature-nurture debates, opposing sides often assume, imply, or propose that these different sciences are speaking to the same issues. This sense of equivalence or, at least, mutual relevance is evident most notably in discussions of heritability, the technical name for genotypic variance (i.e., variation among variety means) as a fraction of the total variation. Heritability has nothing to do with the colloquial view that a trait is heritable when it involves transmission of a gene or genetic factor from parent to offspring. To add error on top of this regrettable ambiguity, researchers who are proficient in type 1 analysis often refer to heritability as the “contribution of genetic differences to observed differences among individuals.” (The quote is from Plomin et al. [1997, 83], but the interpretation is widespread.) Similarly, interpretation of other fractions of variation in terms of differences in yet-to-be identified environmental factors is not warranted.
I have a conjecture that, just as there was a manipulable level of a measured environmental factor in the data used by RA Fisher in his pioneering work on statistical analysis of variation, the components of variation derived from type 1 analysis have been imagined by researchers to correspond to measurable, albeit yet-to-be-identified genetic and environmental factors. It then seemed plausible that the same kinds of factors underlying variation within groups were associated with the variation between groups (strictly, to the difference between the averages for the groups).
Reference: Plomin, R., J. C. Defries, et al. (1997). Behavioral Genetics. New York: Freeman.
(For further elaboration on assertions and terminology used in this post, see Taylor, Peter J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.)