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.
Initial notes on the Cases
From PT:
Get the overall idea, concepts, and evidence first for the 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 2007 casts doubt on the findings that underlie both Turkheimer and Plomin's articles by exposing problems with the methods used to arrive at those findings. (Ideas about genes and environment are mostly ideology unless there's clear definitions of what "genetic" and "environment" mean, there's a reliable method to separate their contributions, and the findings are replicated by a number of researchers.) 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
Caspi 2002 we first saw in week 1. Moffitt 2005 provides a review of what's involved in trying to identify 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.
Substantive statement
Heritability
Heritability is a measure of the degree to which the variance in the
distribution of a phenotype is due to genetic causes. In the broad sense it is measured by the total genetic variance divided by the total phenotypic variance. In the narrow sense it is measured by the genetic variance due to additive genes divided by the total phenotypic variance. Heritability analyses estimate the relative contributions of differences in genetic and non-genetic factors to the total phenotypic variance in a population. Phenotype is the observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences or the expression of a specific trait, such as stature or blood type, based on genetic and environmental influences. Consider a statistical model for describing some particular phenotype: Phenotype (P) = Genotype (G) + Environment (E). Considering variances (Var), this becomes:
Var (P)+Var (G) + Var (E) + 2Cov (G,E)
The parameter H2 is the broad sense heritability and reflects all possible genetic contributions to a population’s phenotypic variance. Included are effects due to allelic variation (additive variance), dominance variation or which act epistatically (multi-genetic interactions), as well as maternal and paternal, where individuals are directly affected by their parent’s phenotype (such as milk production in mammals).
Heritability is a proportion with varying numerical values. Its numerical values can range from 0.0 (genes do not contribute to generic differences and 0 to1.0 ( genes are the only reason for individual differences). Environmentability is the proportion of the phenotypic variance that is associated with environmental variance or they are the extent to which individual differences in the environment augment individual differences in behavior. The heritability of most human behaviors lie in the range of .30 to .60 thus the environmentability of most human behaviors lie in the range of .40 to .70.
Heritability and environmentability are abstract concepts. Heritability estimates do not tell of the specific genes that contribute to a trait and estimates of environmentability provide no information about environment variables that influence behavior. Heritability is a population concept and tells nothing of an individual. Say we are measuring timidity that has a heritability of .40 - this means that timidity may be some way attributable to genetic individuals differences but it does not mean that 40% of a person’s timidity is due to his/her genes and 60% to environment. Heritability depends on a range of typical environments in the population. If the environment is homogenous than heritability may be high but if environments are heterogeneous than heritability may be low.
Heritability of Alcohol Use
In the Finnish twin studies of Kaprio et al., performed in 1981, they evaluated heritability of alcohol use. They found relatively constant heritability for age groups from 18 to 59 (ranging from 0.48 to 0.65) but in the oldest group ( over age 60) heritability was almost zero. Similarly in their Australian sample, Jardine and Martin (1984) found that for males, heritability was much higher in the group aged 18 to 30. A confounding variable is that it is difficult to disentangle cohort effects from developmental effects. In the United States, it has been demonstrated in many studies that that there are significant secular trends in the familiar transmission of alcoholism. It was noted that familial transmission is greatest in younger cohorts. Cultural as well as genetic modes of inheritance are included in the familial parameters in the models studied. It is clear that the increases in familial transmissibility seen are due to cultural, environmental factors, rather than to genetic factors, since the trends tend to occur too rapidly to be associated with genetic change. Cohort differences associated with cultural models must be taken into consideration when evaluating age differences in heritability studies of alcohol use and abuse. In the classic twin method, the differences between intraclass correlation for MZ and DZ twins is double to estimate heritability. The remaining population variation can then be attributed to environmental factors. These factors may include cultural factors, factors shared by relatives living together, or factors that are unique to the individual. Path modeling approaches facilitate the division of the total phenotypic variance into genetic, shared environments and non-shared environmental components.
Hereditary factors appear to operate in normal drinking behavior and in individual vulnerability to alcohol abuse. The flushing reaction to acetaldehyde is a clear instance of a pharmacogenetic variant that influences human behavior. Alcoholism runs in families and is manifest in men more often than in women. The familial preponderance is related primarily to genetic factors, and the differentiation by gender is primarily the result of sociocultural factors. There may be two distinct types of alcoholism with different patterns of inheritance: type I (or milieu-limited) and type II (or male-limited). Type II alcoholism is more severe and more likely to be influenced strongly by major gene effects. Genetic markers for the vulnerability to alcoholism have yet to be verified. Of most interest are studies suggesting EEG/ERP differences in those at risk and studies showing decreased responsiveness to alcohol in those at risk (see annotated reference for details). The neurochemical investigation of animal models of alcoholism is showing links to the serotonin system, as are recent genetic studies. Genetic linkages to areas on chromosomes 1, 2, 4, and 7 and elsewhere are being explored in ongoing studies. From an heritability standpoint, the interplay of the environment with genetics all contribute to the phenotype for alcoholism. (jg)
Annotated additions by students
Xiao Lei Zhang & Howard L. Cohen, Electrophysiological differences of individuals at high risk for alcoholism,
Science Blog: Alcoholism: Clinical & Experimental Research,
http://www.scienceblog.com/community/older/2001/A/200110262.html
Researchers examined a group of young people who are at increased genetic risk for alcoholism as a result of having a high prevalence of alcoholism in their biological lineage. Study authors hypothesize that high risk for alcoholism is related to brain hyper-excitability, including impaired brain inhibitory mechanisms. Brain deficits in chronic alcoholics were studied for many years using brain wave recordings and it was found that some of these deficits do not recover with prolonged abstinence from alcohol. Studies of high risk children of alcoholics, prior to any exposure to alcohol, indicate that many of these electrophysiological anomalies antecede the development of alcoholism. Based on these studies, it was hypothesized that a predisposition to developing alcoholism involves a deficit in central nervous system inhibition. Event-related potentials (ERP’s) are brain responses that manifest as small voltage changes in that can be measured in microvolt. ERP’s consist of positive and negative waves that are linked in time to specific sensory or cognitive events, and are extremely sensitive to ongoing processing in the brain. Mismatch negativity (MMN) is an ERP component that occurs when a series of respective standard stimuli is interrupted by a deviant tone. MMN reflects the involuntary matching of incoming stimuli with the current template in short term acoustic memory. This study shows that mismatch negativity is larger in individuals at increased risk for alcoholism, thereby providing direct evidence for increased brain excitability Individuals manifesting this deficit in inhibition (or hyper-excitability) on the electrophysiological level are also more likely to manifest behavior disinhibition. Both high risk and alcoholic individuals have been characterized as having a disinhibited personality. Behaviorally, this has been manifested as a higher incidence of risk taking behavior, such as drug and alcohol abuse, smoking, risky sexual activity, aggression and antisocial behavior.(jg)
Whitfield, K.E. and McClearn, G. (2005). Genes, environment, and race.
American Psychologist, 60 (1): 104-114.
This is a particularly good article for introducing an integrated approach to genetics, environment, and health from a health disparities perspective. The authors stress the need to use specific operational definitions when discussing genes and race; for example, “genetic differences” are defined for individuals as specific configurations of alleles at the loci under examination. A non-specialist discussion of “what genes really do” follows in which ‘the environment’ is described to be all effective elements in the system that are not derived from DNA – i.e. composed of influences from the physical as well as the sociocultural realms. The field of quantitative genetics is described and the usual caveats about the limitations of heritability are made before the authors turn to specific models of racial/ethnic influences using twin studies. Some very interesting examples of the relative contribution of genetic and environmental factors in determining blood pressure among African Americans are given. Using the Carolina African American Twin Study of Aging data, it was found that although a large portion of individual variability in blood pressure was accounted for by genetic factors (heritabilities of .44 and .52 for diastolic blood pressure and .52 for systolic blood pressure), environmental factors accounted for .56 (DBP) and .48 (SBP) after controlling for the effects of antihypertensive medication. The authors conclude from this that although genes impact blood pressure, environmental factors do play a larger role in determining differences in blood pressure, especially as people age. The article concludes with a call for interdisciplinary approaches to health differentials rather than the “intellectually impoverished nature-nurture” dichotomy. (JC)
Both Plomin and Taylor refer to this report(2002,
UK)by the Nuffield Council on Bioethics regarding genetics and human behavior. Below is a link to the document, which while long, elucidates in well-organized chapters and sections ethical issues (and potential ethical issues) related to the use of genetic information (including their recommendations), such as relation to human dignity, access to genetic testing, monitoring provision of genetic testing, use of genetic information in criminal justice system and application of genetic information in employment, education and insurance. I have just skimmed some of the chapters but one of the messages the authors send is that the field of genetic behavior is by no means complete and does not present a new theory of human behavior (for one due to the complex relationship between genetics and environment)but instead complements current social science....EB
{{http://www.nuffieldbioethics.org/fileLibrary/pdf/nuffieldgeneticsrep.pdf}}
