Previous posts (1, 2, 3) 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.)
The second answer, elaborated on in the previous post, revolved around supposing there were factors common to everyone in group A and other factors common to everyone in group B, with the development of individuals being driven by the common factors and any other factors contributing variation around the essential trajectory. If the common factors were genetic, then understanding the relative degrees of genetic versus environmental influences on the variation of the trait would provide a sense of how fixed a trait is (given that the genes an individual gets at conception are the ones they have throughout life).
A key problem, as for the first answer to the title question (see earlier post), is that the science of partitioning variation in a given trait invoked as relevant to estimating the relative degrees of hereditary versus environmental influences on the variation of the trait within group is not able to do provide such estimates (see previous post). Can we, however, formulate a science that does estimate the relative degrees of genetic versus environmental influences on the variation of the trait? If so, then this might speak to the fixity versus flexibility in the development of traits in individuals over their life course. Let us elaborate on this the third answer to the title question and consider its problems.
1. Just as in the second answer, let us suppose there are factors common to everyone in group A and other factors common to everyone in group B, even though there are heterogeneous other factors that differ between any two individuals (whether in the same group or in the other group). If the development of individuals is driven by the common factors, with the other factors contributing variation around the normal trajectory, then understanding the relative degrees of influence of common factors versus the other factors provides a sense of how much is determined by the common factors.
2. Again, as in the second answer, let us note that there are many aspects of development that occur reliably despite the variation in the rest of development. It is not ridiculous to hypothesize that factors that are common to all chromosomal XY males and those common to all chromosomal XX females drive their different essential trajectories, at least for some aspects of development.
3. Now consider how to estimate the relative degrees of influence of common factors versus the other factors. If every chromosomal XY male has the common factor, then there will be no variation among those males that can be tied to that factor (except in a secondary sense of variation tied to the way other factors combine with that common factor). Ditto for chromosomal XX females.
4. We might, however, imagine combining all individuals into one study and seek to analyze the influence of the difference between the as-yet-unidentified common factors. Given that almost all chromosomal XY males (*) are treated as males by parents and society more generally, any analysis will have difficulty distinguishing the internal common factors (effect of the XY vs. XX chromosomes) from the external ones (effect of being treated as male vs. female), at least for traits that develop or continue to develop after birth. (* Some exceptions are Swyer syndrome XY and transgender individuals.)
5. We might try to get around this last problem by collapsing the external onto the internal, that is, being treated as male follows from being chromosomal XY (with a few exceptions). Ditto for chromosomal XX. Estimating the relative degrees of influence of common factors versus the other factors becomes not hereditary versus environmental, but sex-chromosome-determined versus other environmental factors (e.g., socio-economic status, quality of schooling, nutrition).
6. A number of problems arise:
a) Being treated as male versus female can extend to nutrition, quality of schooling, socioeconomic status as adult, etc.—where is the dividing line between what aspects of the external are assumed to go together with the internal?
b) Being treated as male versus female can change over time and from culture to culture—how is the relative influence of common vs. other factors then going to illuminate fixity vs. flexibility (i.e., the other Nature-Nurture issue)?
c) Estimating the relative degrees of influence of common factors versus the other factors depends on comparing difference in the means to variation around those means. Almost all traits—from the physical, such as height, to the behavioral, such as aggression—show overlap.
d) Even if the overlap were small so we estimated that the common factors has relatively large influence, how would we interpret the variation around the mean in relation to the other Nature vs. Nurture, i.e., fixity vs. flexibility in development over the life course? If we said that where one is on the spectrum, development is fixed, not flexible because common factors have a larger influence than the others, what would explain the variation around the means? Moreover, how would we discount the possibility of heterogeneity of the factors (see discussion of underlying heterogeneity and explaining difference between means on an earlier post).