Heritability, heterogeneity, and group differences
Idea: As conventionally interpreted, heritability indicates the fraction of variation in a trait associated with "genetic differences." A high value indicates a strong genetic contribution to the trait and "makes the trait a potentially worthwhile candidate for molecular research" that might identify the specific genetic factors involved. I contest the conventional interpretation and contend that there is nothing reliable that anyone can do on the basis of estimates of heritability for human traits. While some have moved their focus to cases in which measurable genetic and environmental factors are involved, others see the need to bring genetics into the explanation of differences among the averages for groups, especially racial groups.
Guidelines for annotations
Notes and annotations from 2007 course
Initial notes from PT
Common readings = Taylor 2009 (skepticism about the estimation and interpretation of heritability); Moffitt 2005 (Interaction of measured genes and measured environments)
There are many supplementary readings. Get the overall idea, concepts, and evidence first for these articles, going back to go through the equations only if you have time and aptitude/perseverance.
a. Heritability & critique
Heritability is a quantity derived from analysis of variation in traits of humans, other animals, or plants in ways that take account of the genealogical relatedness of the individuals whose traits are observed. Such "quantitative genetic" analysis does not require any knowledge of the genes or "measurable genetic factors" involved.
Turkheimer is "on the left" of behavioral genetics, being much less gung ho about the implications of its findings. Here he gives a clear overview of what the field has shown.
Plomin articulates the confident consensus of behavior genetics, namely, that they've debunked the supposed environmentalist orthodoxy in social science that says that everything is social and have established a basis for connecting with molecular genetics to identify the actual genetic factors.
Rutter, a senior psychological researcher (who once worked with Brown on social determinants of mental illness), tries to moderate the "polarizing claims" and "unwarranted extrapolations."
Taylor 2009 casts doubt on the findings that underlie both Turkheimer and Plomin's articles by exposing problems with the concepts and methods used to arrive at those findings. Taylor ends with a nudge towards methods that use measured genetic factors as well as measured environmental factors (the latter being the staple of social epidemiology).
b. Interaction of measured genes and measured environments
Moffitt 2005 provides a review of what's involved in trying to identify interactions between measured genetic and environmental factors. (Use Taylor 2009 to get clear about the difference between this kind of interaction and the classical genotype x environment interaction in quantitative genetics.) Caspi 2002 is one of two 2002 papers that caused a lot of splash. Davey-Smith picks up on the current consensus that the 2002 studies have been hard to replicate and invokes Mendelian randomization as a way to strengthen causal inference about interactions between measured genetic and environmental factors.
c. Data & models about heritability & change (or lack of it)
Dickens 2001 provides a resolution of the paradox that heritability of IQ test scores is reported to be high, but there has been a large increase in average IQ test scores from one generation to the next. We know that genes haven't changed from one generation to the next, so Dickens' account is also exposing a flaw in the logic that because heritability of IQ test scores is high within racially defined groups and because there is a large difference in average IQ test scores between whites and blacks, genetic factors are probably involved in that difference.
Rushton 2005 however thinks that 30 years of research has validated that idea.
Taylor 2009 refers to Dickens 2001, but gives a somewhat different spin on its implications.
Annotations on common readings
Heritable: (capable of being inherited) "inheritable traits such as eye color"; "an inheritable title"
http://wordnetweb.princeton.edu/perl/webwn?s=heritable
Heritability: A measure of the strength of the relationship between performance (phenotypic values) and breeding values for a trait in a population. (http://www.alpacas.com/AlpacaLibrary/GlossaryGL.aspx)
According to Gordis, a disease may occur in hereditary and non-hereditary forms. Variation in age of onset of disease is often the result of this distinction, with familial or genetic influence leading to disease development at a much earlier age. Gordis explains further that in cases where environment plays a primary role in disease development, in the case where there is not an underlying genetic factor, there has to be a cumulative effect, and thus a longer timeframe (i.e. older age of onset) for development. He notes the challenge in separating genetic and environmental factors in disease, as families typically share both (holds true for twins but not for spouses). Twin studies, adoption studies and migrant studies provide naturally occurring experiment opportunities for G&E researchers. He concludes the chapter by noting the value of combining epidemiologic and molecular methods to observe and understand the interaction of genes and environment.
Strategy for Investigating Interactions Between Measured Genes and Measured Environments (Moffit et al) Gene-Environment Interaction (G&E)
This article looked at environmental and genetic causation of mental disorders, specifically; the relationship of childhood abuse to later violence, the effect of stressful life events on depression, and the possibility of smoking marijuana leading to psychosis. The authors outlined seven strategic steps for research pertaining to G&E.
1) Using quantitative models of data from twin and adoption studies
2) Expand conceptualization of environmental factors to include perinatal effects, infectious and pathogenic factors, in addition to the more traditional psychosocial. This should be done in combination with controlling for genetic effects, again relying on experimental, longitudinal quantitative studies of twins and adoptees.
3) Measuring environmental factors accurately and differentiating between proximal and distal factors. Looking a cumulative, life-course effects. Retrospective, longitudinal studies are recommended.
4) Selecting genes for examination critically, along the criteria outlined in the article.
5) Importance of testing for interactions. Again, a longitudinal study is recommended, among a cohort sample.
6) If G&E interaction found, determining relationship between gene, environmental factor and disorder.
7) Final step includes replicating findings through a meta-analysis to ensure finding is not just a fluke.
This article concludes by highlighting the importance of looking at both nature and nurture, and the influence of one on the other. The researchers recommend future studies including individuals with the same disorder, where there is variation in genetic susceptibility and environmental factors.
Three puzzles and eight gaps: what heritability studies and critical commentaries have not paid enough attention to
(Taylor) Quantitative Genetics (QG)
This article is critical of the field of QC and some long-standing assumptions. The argument is laid out along the lines of gaps and puzzles. The author questions the applicability of heritability studies to human behavior and traits. This article left me with the question of: what is mean by the recommendation to “keep the gap” open? To keep the discussion open and moving forward? To leave the gap unresolved? I’ll briefly summarize the gaps (in terms of the recommended appropriate responses) here:
1) Clarify distinct meanings of key terms in QG.
2) Highlight that statistical patterns differ from hypotheses about measurable factors.
3) And subsequently, translation from statistical patterns to hypotheses about measurable factors is difficult.
4) Compensate for discrepancies in predictions, in which extrapolations from existing patterns may not match outcomes.
5) Limitations in being able to study variation in human studies, due to limited size of sample (esp. twin and adoption studies).
6) Underlying heterogeneity (dissimilarity, disparateness) of genetic and environmental factors further complicates translations from statistical analyses to hypotheses about measurable factors.
7) Recognize estimates of heritability, based on claims of causal influence of genetic or environmental factors have limited utility.
8) Variation within groups does not translate to explain differences among groups.
The issue of underlying heterogeneity is one of the puzzles), as is generational increases in IQ scores (controversy re: racial group mean scores), and also how selective breeding of animals and plants has shaped understanding and research regarding human genetics.
Interesting supposition that genetic does not mean unchangeable, as many people do tend to assume that one is born with genes, and that’s the hand that has been dealt. One must also consider the occurrence of heritability within a generation, versus being restricted to inter-generational transmission. Shifting the focus off of humans exclusively, to incorporate other species enables the conceptualization of other factors and patterns, i.e. less restrictive thinking. This recommendation is outlined in a table that compares the limitations of analyzing data exclusively among humans as compared to agricultural trials. The author argues in favor of using the more neutral term of ‘variety’ and ‘location’ over genotype and environment which have more loaded connotations. Moreover, readers are cautioned against mistakenly thinking that variety and location can be replicated, with respect to human traits.
In sum, the framework of the aforementioned gaps is intended to question the usefulness of using classical methods to identify and understand genetic and environmental factors. By challenging the many assumptions that QC studies and findings rest upon, is to begin to solve the puzzles and paradoxes outlined within and across groups, generations, species, and the underlying heterogeneity of varieties and locations. The author compares to QC to alchemy, in that both rest upon a scientific approach and yet posed significant limitations to true understanding and progress in their respective domains of scientific inquiry.
posted by Amy Helburn November 20th 2009
Annotated additions by students
Three Laws of Behavior Genetics and What They Mean (Turkheimer, 2000)
According to the author the long gone nature-nurture debate is over, and he starts his article with the conclusion that “everything is heritable” meaning that nature has won over nurture. However, Turkheimer delineates three laws, with the third law suggesting that winning was not a clear-cut: 1) that all human behavioral traits are heritable, 2) that the genes have stronger effect than being raised in the same family, and 3) that a substantial portion of the variation is due to “nonshared environment”, meaning not due to the effect of genes or families. In the attempt to illustrate the process of person’s development, he shows that the main feature of the process is the interactivity, where everything is interacting with everything. The nonshared environment leads to the new big concern that shifted from what children inherit and share to what they do not share and why. The gloomy project, as they call it, refers to the substantial portion of nonshared environmental variability. The next step to come out of this (or to overcome the gloominess) is to focus on identifying specific genes or domains of genetic causation that could help explain different behavioral outcomes. I wonder how much success this attempt has made since the 2000 when the goal was set (DBJ ’09)
