What has Nature vs. Nurture got to do with Nature vs. Nurture? III

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.) 

One answer, elaborated on in that post, depends on making three assumptions, which a subsequent post showed were all unreliable.   In light of that, what, we might ask, leads people to make the assumptions (perhaps unwittingly) and see the two Nature-Nurture issues as related? Before addressing that question, let us consider a slightly different answer to what the two Nature-Nurture issues have got to do with each other.

The second answer, which the rest of the post dissects:
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 essential (*) 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.  If the common factors are genetic, then understanding the relative degrees of genetic versus environmental influences on the variation of the trait provides a sense of how fixed a trait is given that the genes an individual gets at conception are the ones they have throughout life.

Elaboration of the answer and its problems

0. * A typological or essentialist worldview “conceptualizes diversity as ‘deviation’ from a natural state or path of change” [McLaughlin 1998, 25].

1. There are many aspects of development that occur reliably despite the variation in the rest of development. For example, most of us have four limbs and digits at the end of those limbs. When exceptions exist, the perturbation lies outside the normal range of factors and can often be isolated, e.g., thalidomide taken early during the mother’s pregnancy, roadside explosives, polydactyly gene.  In some instances no exceptions and corresponding perturbations exist.  For example, humans and chimpanzees differ in 1.2% of their DNA bases; no chimp develops to look like a human or vice versa.

2.  It is not ridiculous to hypothesize that factors that are common to all chromosomal XY males drive their essential (*) trajectory.  Similarly for all chromosomal XX females.  Differences in the common factors then explain their different essential trajectories, at least for some aspects of development.

3.  The common factors could be genetic, but they could also be environmental or a combination in which being driven by common genetic factors depends on the appropriate environment.  (On this last possibility, Hendriks-Jansen (1996, 1997) for example, provides an “interactive emergence” view of very early infant development, in which a sense of agency emerges out of parental responses to fixed action patterns of the infant.)  There’s a catch-22 here: One almost needs to know how development works, at least the reliable aspects of it, before one can tease apart—or tease out—the contributions of genetic and environmental factors to the dynamics of development.

4.  In the absence of an account of development, it might be thought that knowing the relative degrees of hereditary versus environmental influences on the variation of the trait within group A would indicate the degree to which the common factors were genetic.  Ditto for within group B.  In that way, the Nature vs. Nurture of relative degrees of influence would speak to the fixity versus flexibility in the development of traits in individuals over their life course.

5.  The 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 that (see previous post).  (Here only the first two assumptions—a genetic gradient underlying the variation in the trait and homogeneity of underlying factors—are being made—and then called into question.)

6.  Notice that, for the second answer, the issue of the relative degrees of hereditary versus environmental influences within versus between groups has not come into play.  A subsequent post takes that up in considering a third answer to what Nature vs. Nurture has got to do with Nature vs. Nurture.

References

Hendriks-Jansen, H. (1996). “Situated robotics, natural selection, and cultural scaffolding: the ingredients of a historical explanation”. Pp. 1-15 in Catching Ourselves in the Act: Situated Activity, Interactive Emergence, Evolution, and Human Thought. Cambridge, Massachusetts: The MIT Press

Hendriks-Jansen, H. (1997). “The epistemology of autism: Making a case for embodied, dynamic and historical explanation.” Cybernetics and Systems 28: 359-415.

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