Three Laws of Behavior Genetics and their Meaning
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Three Laws of Behavior Genetics and their Meaning
Abstract Behavior genetics has demonstrated that genetic variance is an important component of variation for all behavioral out- comes, but variation among families is not.
These results have led some critics of behavior genetics to conclude that heritability is so ubiquitous as to have few consequences for scientific understanding of development, while some behavior genetic partisans have concluded that family environment is not an important cause of developmental outcomes.
Both views are incorrect. Geno- type is in fact a more systematic source of variability than environment, but for reasons that are methodological rather than substantive. Development is fundamentally nonlinear, interactive, and difficult to control experimentally. Twin studies offer a useful methodological shortcut, but do not show that genes are more fundamental than environments.
Keywords genes; environment; development; behavior genetics
The nature-nurture debate is over. The bottom line is that every- thing is heritable, an outcome that has taken all sides of the nature- nurture debate by surprise.
Irving Gottesman and I have suggested that the universal influence of genes on behavior be enshrined as the first law of behavior genetics (Turkheimer & Gottesman, 1991), and at the risk of naming laws that I can take no credit for discovering, it is worth stating the nearly unanimous results of behavior genetics in a more formal manner.
c First Law. All human behavioral traits are heritable.
c Second Law. The effect of being raised in the same family is smaller than the effect of genes.
c Third Law. A substantial portion of the variation in complex hu- man behavioral traits is not ac- counted for by the effects of genes or families.
It is not my purpose in this brief article to defend these three laws against the many exceptions that might be claimed. The point is that now that the empirical facts are in and no longer a matter of serious controversy, it is time to turn attention to what the three laws mean,
to the implications of the genetics of behavior for an understanding of complex human behavior and its development.
VARIANCE AND CAUSATION IN BEHAVIORAL
If the first two laws are taken literally, they seem to herald a great victory for the nature side of the old debate: Genes matter, families do not. To understand why such views are at best an oversimplification of a complex reality, it is necessary to consider the newest wave of opposition that behavior genetics has generated.
These new critics, whose most articulate spokes- man is Gilbert Gottlieb (1991, 1992, 1995), claim that the goal of developmental psychology is to specify the actual developmental processes that lead to complex outcomes.
In lower animals, whose breeding and environment can be brought under the control of the scientist, it is possible to document such developmental processes in exquisite de- tail. The critics draw an unfavorable comparison between these detailed animal studies and twin studies of behavior genetics, which produce only statistical conclusions about the relative importance of genes and environment in development.
The greatest virtue of the new challenge is that it abandons the
Three Laws of Behavior Genetics and What They Mean Eric Turkheimer1 Department of Psychology, University of Virginia, Charlottesville, Virginia
160 VOLUME 9, NUMBER 5, OCTOBER 2000
Published by Blackwell Publishers Inc.
implausible environmentalist contention that important aspects of behavior will be without genetic influence. Gottlieb (1992) stated, “The present . . . viewpoint holds that genes are an inextricable com- ponent of any developmental system, and thus genes are involved in all traits” (p. 147).
Unlike earlier critics who deplored the reduction- ism they attributed to behavior genetic theories of behavior; the developmental biologists take behavior genetics to task for not being mechanistic enough.
Once vilified as the paragon of determinist accounts of human behavior, behavior genetics is now chastised for offering vague and inconclusive models of development (Gottlieb, 1995; Turkheimer, Goldsmith, & Gottesman, 1995), and judged by the standards of developmental psychobiology in lower animals, it
is true enough that behavior genetic theories of complex human behavior seem woefully poorly specified. But ultimately the charge is unfair, because there is no equivalent in developmental psychobiology to the behavior genetic study of marital status or school performance. The great preponderance of the exquisite experimental science that goes into animal psychobiology is quite simply impossible to conduct in humans.
Figure 1 is a schematic illustration of the problem. Individual genes (Genes 1, 2, and 3) and their environments (which include other genes) inter- act to initiate a complex develop- mental process that determines adult personality. Most characteristic of this process is its interactivity.
Subsequent environments to which the organism is exposed depend on its earlier states, and each new environment changes the develop- mental trajectory, which affects future expression of genes, and so forth. Everything is interactive, in the sense that no arrows proceed uninterrupted from cause to effect; any individual gene or environ- mental event produces an effect only by interacting with other genes and environments.
For the behavior geneticist,
Fig. 1. Schematic diagram of contrasting roles of genes and environment in development of personality. One-headed arrows link causes to effects; two-headed arrows indicate correlations.
Genes and environments are both causal inputs into an interactive developmental system (represented by the network of arrows in the center of the figure), but because people select and shape their own environments (as represented by lighter one-headed arrows from personality to environments), correlations across the developmental system (dotted two-headed arrows) are easier to detect for genes than for environments.
161CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
however, the quasi-experimental gift of genetically identical and nonidentical twins offers a remark- able, if deceptively simple, method to span this daunting interactive complexity.
Thanks to the fact that identical twins are on average exactly twice as similar genetically as nonidentical twins, one can use straightforward statistical procedures to estimate the proportion of variability in complex outcomes that is associated with causally dis- tant genes, all the while maintaining a state of near-perfect ignorance about the actual causal processes that connect genes to behavior.
This methodological short- cut is not available to rivals of be- h a v i o r g e n e t i c s w h o s e e k t o measure the effects of families on behavior. How similar was my rearing environment to that of my siblings? And how similar was it to the environment of my adopted sibling, if I have one, or to the environment of my biological sibling who was raised by someone else? The apparent victory of nature over nurture suggested by the first two laws is thus seen to be more methodological than substantive.
In a world in which there were occasional occurrences of “identical environmental twins,” whose experiences were exactly the same, moment by moment, and another variety who shared exactly (but randomly) 50% of their experiences, environmentalists could re- produce the precision of their riv a l s , a n d l i k e t h e b e h a v i o r geneticists could measure with great precision the total contribution of the environment while knowing almost nothing about the developmental processes that underlie it.
The old-fashioned nature-nurture debate was about whether or not genes influence complex behavioral outcomes, and that question has been decisively answered in the affirmative.
The new question is how we can proceed from partitioning sources of variance to specifying concrete developmental processes (Turkheimer, 1998), and although critics like Gottlieb are correct that heritability per se has few implications for a scientific understanding of development, they have failed to emphasize two crucial points.
First, heritability does have one certain consequence: It is no longer possible to interpret correlations among biologically re- lated family members as prima fa- cie evidence of sociocultural causal mechanisms. If the children of de- pressed mothers grow up to be de- pressed themselves, it does not necessarily demonstrate that being raised by a depressed mother is it- self depressing.
The children might have grown up equally depressed if they had been adopted and raised by different mothers, under the influence of their biological mother’s genes. For every behavior geneticist who continues to report moderate heritabilities as though they were news, there is an environmentalist who reports causally ambiguous correlations between genetically related parents and children.
Second, the problem the critics have uncovered extends well beyond behavior genetics: It is a rare environmentalist who has never used statistical methods to predict behavioral outcomes from earlier events, in the hope that the specific developmental mechanisms can be filled in later.
The dis- connect between the analysis of variance and the analysis of causes, to use Lewontin’s (1974) phrase, is not a proprietary flaw in behavior genetic methodology; in fact, it is the bedrock methodological problem of contemporary social science.
NONSHARED ENVIRONMENT AND THE
Even after the effects of genes and the shared effects of families
have been accounted for, around 50% of the differences among sib- lings is left unexplained. In recent years, scientists interested in the genetics of behavior have come to call this unexplained portion the “nonshared environment.” Al- though according to the second law shared environment accounts for a small proportion of the variability in behavioral outcomes, ac- cording to the third law, nonshared environment usually accounts for a substantial portion.
So perhaps the appropriate conclusion is not so much that the family environment does not matter for development, but rather that the part of the family environment that is shared by siblings does not matter.
What does matter is the individual environments of children, their peers, and the aspects of their parenting that they do not share. Plomin and Daniels (1987) reviewed evidence of the predominance of nonshared environmental variance and posed a seminal question: Why are children in the same family so different?
They proposed that siblings are different because nonshared environmental events are more potent causes of developmental outcomes than the shared environ- mental variables, like socioeconomic status, that have formed the traditional basis of sociocultural developmental psychology.
Plomin and Daniels’s explanation involves a subtle conceptual shift, best described in terms of a distinction between the objective and effective environment (Gold- smith, 1993; Turkheimer & Waldron, 2000). What qualifies an environmental event as nonshared? There are two possibilities. The first is objective: An event is non- shared if it is experienced by only one sibling in a family, regardless of the consequences it produces. The other possibility is effective: An environmental event is non- shared if it makes siblings different
Three Laws of Behavior Genetics and their Meaning
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