Practice run of a talk to philosophers of biology & biologists, March 2016 Continue reading
PKU (Phenylketonuria) is a condition that is often invoked to demonstrate that genetic does not mean unchangeable. It is also often cited as a trait with 100% heritability in normal environments that can, nevertheless, be changed and in the right environment have zero heritability. This post explains why these two sentences are not synonymous and how the second is flawed and reinforces an incorrect idea of heritability. I also examine PKU in terms of measured genetic and environmental factors, arriving at the counter-intuitive idea that no gene x environment interaction is involved. Continue reading
The first installment ended on the following note: Suppose you have many sets of same-sex non-identical twins raised together and many sets of same-sex identical twins raised together and find that the identical twins are on average more similar. It seems reasonable to conclude that is because they share all their genes whereas the non-identical twins share fewer of their genes. Reasonable, but not certain, at least not certain that it is only about genes. After all, the treatment of identical twins could be more similar than the treatment of non-identical twins, even same-sex non-identical twins. In any case, that conclusion doesn’t say that it’s the same nature—the same genes—or the same nurture that brings about the resemblance from one pair of twins to the next. Given this possibility of underlying heterogeneity where are you? What can you do? Continue reading
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.)
A previous post considered the connection between two different Nature-Nurture issues: the matter of fixity versus flexibility in the development of traits in individuals over their life course and the relative degrees of hereditary versus environmental influences on the variation of the trait between versus within groups? (“Groups” here refers to males or females, but the question might be extended to socially defined racial or socio-economic groups.) Continue reading
The previous post asked about the connection between two different Nature-Nurture issues: the matter of fixity versus flexibility in the development of traits in individuals over their life course and the relative degrees of hereditary versus environmental influences on the variation of the trait between versus within groups? (“Groups” here refers to males or females, but the question might be extended to socially defined racial or socio-economic groups.) Continue reading
How difficult is it to change the typical distributions of a trait, such as aggression, substance abuse, suicide attempts, as the distributions differ between males and females? Nature versus Nurture debates build off this question in two ways. One is the matter of fixity versus flexibility in the development of traits in individuals over their life course. The other is the relative degrees of hereditary versus environmental influences on the variation of the trait between versus within groups. (“Groups” here refers to males or females, but the question might be extended to socially defined racial or socio-economic groups.) What have these two Nature-Nurture issues got to do with each other? Continue reading