Behavior Genetics Today

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The aim of behavior genetics was set forth clearly in the first issue of the journal Behavior Genetics in 1970: ''. . . behavior genetics is simply the intersection between genetics and the behavioral sciences.'' Although from the beginning, human behavior was considered an important component of the field, many investigators focused their attention on less complex animals, especially the small fruit fly D. melanogaster, since so much was known about its genetics. Pioneers in that work included American psychologists-geneticists such as A. Manning and Jerry Hirsch, both of whom carried out breeding and selection experiments on Drosophila to study the mode of inheritance of behavior and its modification through selection. For example, in 1961 Manning carried out two selection experiments for mating speed in D. melanogas-ter, producing a slow and a rapid strain that were easily distinguishable. Hirsch selected for various behavioral traits over hundreds of generations, producing strains with widely divergent behavior patterns. Such experiments showed clearly that specific behaviors in fruit flies have a distinct genetic component and that the traits can be altered by natural selection. Manning, Hirsch, and others also showed that mating behavior in Drosophila, like response of worker bees to dead larvae, consists of a number of separable components.

In the 1980s and 1990s Ralph Greenspan and his associates at New York University identified these components and traced out their neurological and genetic bases. Gene mutations have been observed to affect such features of courtship as the ''mating song,'' produced as sound pulses when the male flaps his wings in certain rhythms. Males with a mating song gene mutation called ''period'' flap their wings at different intervals than normal males. The result is that mutant males have less success copulating with females than normal males do. The ''song'' is just one aspect of a complex courtship ritual that involves specific male and femal motions, wing positions, extension of the proboscis (a long feeding device extruded from the mouth), licking of the genitals, and copulation itself. That these individual components can be isolated and studied at the genetic and neurological level demonstrates the power of behavior genetic analysis when carried out under rigorous laboratory conditions.

More recent work has focused on interspecific comparisons in Drosophila, again using mating behavior. Behavioral hybrids produce courtship responses that are often intermediate between the two parental species, thereby reducing the number of successful copulations. Such experiments have thrown much light on Darwin's hypothesized mechanism of sexual selection. A variation of natural selection, ''sexual selection,'' was introduced by Darwin to explain the almost-universal sexual dimorphism (distinctly different) in both physical traits (male-female differences in coloration or plumage in birds, or hair distribution in mammals) and behaviors (maternal versus territorial behavior in female and male baboons, respectively). Such persistent differences of forms within a species seemed difficult to account for in terms of traditional natural selection. Darwin concluded that females choose among competing males for a partner, selecting the male with, for example, the brightest plumage or the most distinct courtship behaviors. Thus, from an original population of undistinguished males in the population, through sexual selection males of many species evolved distinct male-associated characteristics (comb of the cock, mane in male lions, etc.) that seemed to serve no other function than as an attraction to the opposite sex. By way of such arguments, behavior genetics, like its ancestor ethology, has always had a close association with the study of evolutionary processes.

Modern-day behavioral geneticists work with a much larger variety of animals than fruit flies and round-worms. Many studies have been carried out with mice, especially those laboratory strains whose genetics is thoroughly understood. And despite the difficulty in working with them, dogs continue to be a popular object of behavioral studies. Primates such as chimpanzees or macaques have also been used, though for obvious reasons (like humans they have small numbers of offspring and their gestation periods are long) genetic data are more difficult to come by.

In the past decade behavior genetic research on humans has increased at a great rate, despite the conten tiousness and public sensitivity to the issue. Behavior geneticists such as Robert Plomin at Pennsylvania State University and Joel Gelernter at Yale University argue that the field has been misrepresented. They point out that many human behavioral genes, or at least chromosomal or molecular markers thought to be associated with specific genes, have been correlated with specific behavioral types such as Tourette's syndrome (leading to uncontrollable movements and speaking), schizophrenia, manic depression, alcoholism, attention deficit hyperactivity disorder (ADHD), and homosexuality, to name just a few. These correlations suggest strongly that there might be a significant genetic component to these behaviors. Human behavior genetic researchers emphasize that they do not discount the role of environment, nor the additive effect of many genes impinging on any given behavior. In fact, they make a point of emphasizing that the outcome of any behavioral development in humans (or any other organism) is of necessity the product of genes interacting with environment. They therefore argue that the old nature-nurture dispute is meaningless. All traits, including behavioral ones, are a product of the combined effects of heredity and environment.

Since 1998 one of the areas of human behavior genetics to make the most significant progress in trying to pin down genetic influences is research in schizophrenia. Researchers Robert Friedman, Sherry Leonard, and their colleagues at the University of Colorado Medical Center in Denver have studied a number of what are known as nicotinic receptors in the surface of cells in the brain and central nervous system. These receptors are involved in mediating a wide variety of behavioral responses, since nicotinic receptors are among the most common in the central nervous system. There are over a dozen different types of nicotinic receptors, but the researchers have identified a mutant repeat region on chromosome 15 that affects the portion of the gene that enables it to be active (the promoter region). Non-schizophrenics have two copies of this duplicated region while schizophrenics have only one. The studies show that people who have the promoter mutation have a 50% chance of getting schizophrenia while those who lack it have only a 3% chance. Both figures suggest that having the mutation does not ensure that schizophrenia will develop, nor does lacking the mutation mean that the patient will never experience schizophrenia. As in all such gene-behavior relationships, many factors interact to produce a given outcome. Researchers acknowledge, for example, that even with the gene mutation present some traumatic event (physical or psychological) is usually necessary for schizophrenia, or at least some of its symptoms, to develop. In this case, at least, a gene, rather than merely a marker, has been localized that is known to code for a protein that appears to have a distinct behavioral effect.

Basic methods of research in human behavior genetics usually begin with a definition of the behavioral trait in question—alcoholism, schizophrenia, violent/aggressive behavior, homosexuality—followed up by determining criteria for diagnosis (i.e., guidelines for identifying who does and does not display the condition). For example, in genetic studies of crime and alcoholism, psychiatrist C. Robert Cloninger at Washington University Medical School used police and temperance board records from Sweden (where good public health records have been kept for over a century) to classify individual subjects as either ''criminals'' or ''alcoholics'' or both (he used the existence of three or more citations in the public record to establish that an individual was an ''alcoholic'' or ''criminal''). Many psychiatrists in the United States and abroad use the American Psychiatric Association's Diagnostic and Statistical Manual IV (now in the second version of its fourth edition) as the criterion for diagnosing individuals with one or more mental illnesses. Behavioral geneticists emphasize that it is necessary to establish unambiguous definitions of traits before setting out to investigate their inheritance patterns.

A second step is to trace the occurrence of the trait in a given family or group of relatives. The traditional method for recording such data is by constructing a family pedigree chart for the trait through as many generations as possible; in more recent times researchers carry out genetic analysis by identifying chromosome or DNA markers. In genetics a ''marker'' is some detectable region of a chromosome that appears more frequently in individuals who possess a particular trait than in individuals who do not possess the trait. Cytological markers are visible under the microscope (a physical protrusion from the chromosome or special banding pattern observed in chromosome preparations). Molecular markers are segments of DNA detected by special molecular probes. In either case, the presence or absence of the marker is correlated with the presence or absence of the trait in the individual's behavior (the Xq28 marker on the human X-chromosome was correlated with homosexual behavior in 33 out of 40 pairs of gay brothers in a study by Hamer and colleagues in 1993). It is important to note that markers are not equivalent to genes affecting the trait; they only provide some sort of clude about a region of the chromosome where the gene or genes for the trait might be located.

A third component of the method is to analyze statistically the frequency of correlation between marker and visible trait in the family in question, in collateral family lines, and in the population at large. A useful tool for this purpose is known as the analysis of variance, and an associated calculation is the heritability of a trait.

While some behavior geneticists today have abandoned heritability, it has been a staple of human behavior genetics for over fifty years. Heritability is a technique that attempts to partition that part of a trait that is affected by heredity from that which is affected by environment, so that a heritability value of 0.8 (= 80%) can be interpreted to say that 80% of a given trait might be ascribable to hereditary effects. It does not say that 80% of the trait is genetically determined. The term ''heritability'' in its technical sense has led to considerable confusion in the literature, both scientific and popular, since it is often interpreted to mean ''inherited,'' as in ''trait X is 80% due to genetics and 20% due to environment.'' Heritability calculations only state that, all other things being equal, a given heritability (say, 0.8) means that 80% of the trait might be ascribed to genetics. It is, of course, the ''all other things being equal'' part (i.e., knowing the genetic relationships between organisms in the sample, and knowing specific and relevant features of the environment in which the individuals have developed) that is the catch. For human behavior, the latter set of conditions are particularly difficult to assess with any accuracy. (Do two children brought up in the same household have the same environment? Do children brought up in different households have significantly different environments? What counts as significant components of the environment in terms of effects on adult traits?) The statistical analysis part of human behavior genetics has always raised sticky methodological issues.

Finally, once the data are analyzed the behavior geneticist is faced with trying to draw some conclusions about the degree to which a given trait might be affected by some genetic component. This is where even the most staunch behavior geneticist admits there are great pitfalls. The most common is the tendency to overinterpret the data. Are there any genetic effects to be discerned at all? If so, do they appear to be single-gene effects (very few complex traits in humans or any other animals appear to be attributable to single genes), are they additive effects (the presence of two genes yields roughly twice the effect of one), or are they nonadditive (two genes yield three times the effect of one, and so on)? Are the relative comparisons (control groups) available for judging the possible genetic effects in the observed group? For example, if a particular chromosome marker is found in a group of people who show a particular behavioral trait, it would be necessary to know the prevalence of the marker in the general population to draw any conclusion about the possible genetic effects in the observed group.

Critics of human behavior genetics such as Jonathan Beckwith of Harvard Medical School, Peter Breggin, Director of the Center for the Study of Psychiatry

(Bethesda, MD), and Steven Rose at the Open University in Great Britain disagree with the idea that the findings of behavior geneticists are conclusive in any way. They point to a number of methodological problems that have undermined virtually all studies purporting to have found a genetic determiner for any specific human behavior. The flaws generally fall into the four methodological areas dicussed above.

First is the problem of defining human behavioral traits in such a way that they can be diagnosed by any well-trained observer. This becomes difficult, the critics point out, for traits such as criminality, aggressiveness, alcoholism, manic depression, schizophrenia, homosexuality, and many others. These are very complex behaviors and judging whether an individual really fits into the trait category can be quite subjective—was Robin Hood a criminal or a hero when he stole from the rich to give to the poor? Psychiatrists are debating today whether schizophrenia and manic depression are really different diseases or varying manifestations of the same disease. Furthermore, the case where some real genetic differences may exist between individuals—for example, some people can metabolize alcohol more readily than others, and thus exhibit a much greater tolerance— may not be the same as that of social behavior or trait (in the example, ''alcoholism''). With such wide areas of possible disagreement about what constitutes a particular trait, critics point out that it is no wonder behavior geneticists have a difficult time even replicating each other's work, much less carrying out a clear genetic analysis.

At the second step—gathering data on families, siblings, adoptees, etc.—critics argue that many human behavior genetic studies are faulty in a number of different ways. Some have too small a sample size, a special problem for those studies using monozygotic (identical) twins raised apart (Cyril Burt, on whose famous twin studies so much research on the inheritance of IQ has been based, managed after 40 years to accumulate only 53 pairs of twins). Others do not institute proper controls. A 1994 study by Dean Hamer and colleagues at the National Cancer Institute found a genetic marker on the X-chromosome in 33 out of 40 pairs of gay brothers, suggesting to the team that there might be genes located on the chromosome near that marker predisposing those individuals to homosexual behavior. The study did not provide information on whether other brothers (non-gay) in the same families did or did not have the marker. If other sons in the families had the marker, the correlation with homosexuality would be meaningless. Other problems with this second step include non-standardized methods of determining which individuals have the trait in question (different diagnostic procedures or assessments made under different conditions), and bias in selecting subjects for the study (many studies get their subject by asking publicly for volunteers, which can bias the results toward individuals who are outgoing personality types or might exaggerate claims about themselves to remain in the study).

Critics also argue that in the third step of human behavior genetics research—introduction of various statistical procedures—researchers often misuse or misapply particular techniques. One of the most commonly misused and misunderstood statistical procedures is that of heritability. As pointed out above, heritability does not mean ''inherited,'' though many behavior geneticists do not make the distinction clear, especially when talking to reporters or giving popular talks. A more common problem, however, is the failure of those using heritabil-ity to take into account two underlying constraints on the method. First, any heritability estimate is limited to the given population, in a given environment. Thus, the heritability estimate for a trait in population A cannot be applied to population B, since there is no guarantee that either the genetic or the environmental components of the two populations are comparable. Critics point out that Berkeley psychologist Arthur Jensen committed this error in his famous paper of 1969, in which he applied heritability estimates of IQ based on British Caucasian students (population A) to American Caucasian and African-American students (populations B1 and B2). Such a comparison is invalid by the rules of heritability analysis. A second assumption of heritability is that all members within a single population (for example, population A) share the same or nearly similar environments. Since heritability as a technique was introduced in the 1930s primarily as an aid to animal and plant breeders, the assumption, under most breeding conditions, was reasonably safe. However, applied to humans—for example, to Caucasian and African-American populations in any large American city—the assumption is clearly untenable. In addition to these more esoteric statistical procedures, many human behavior genetic studies have been based on extremely small sample sizes, a feature that makes all statistical analysis meaningless. One study of homosexuality consisted of a total of five twin pairs—three male and two female. Even a study of 40 siblings, as in the example of Hamer's research on homosexuality, is tiny by comparison to the number of organisms used in any behavior genetic study of nonhuman animals.

It is in the fourth step or procedure in human behavior genetics—drawing conclusions from the data—that critics find some of the most flagrant violations of sound scientific procedure. On the one hand the conclusions can sound impressive but be enormously trivial. As critics have put it, to claim that a particular study shows that genes and environment interact in producing a par ticular behavioral trait is such a truism as to say nothing of any interest. Every trait—physical, chemical, or bio-logical—in every organism is the result of some interaction between genetic and environmental components. The important question in any given case is to show how and under what conditions (genetic and environmental) a particular behavioral outcome will result. Few if any human behavioral genetic studies have been able to make such a relationship clear or precise. A second problem is in a sense the flip side of the same coin: overinterpretation of the results of a given study. Both Jensen's study of racial differences in IQ in the 1960s and Hamer's study of the hereditary component of male homosexuality in the 1990s were guilty of overinterpretation. Whether in the verbal form of ''the gene for . . .''or ''the genes for . . . ,'' overinterpretation gives the impression, especially among lay readers, that the biological evidence is much stronger than it is. According to most critics of human behavior genetics research, given all the problems with carrying out research on this topic, virtually all strong claims are guilty of overinterpretation.

One aspect of human behavior genetics work that has also bothered critics, as well as other members of the scientific community at large, is the tendency of researchers to talk freely, and often in exaggerated terms, to the press. While announcing new scientific findings in press conferences has become more prevalent in all areas of science than it used to be, it is particularly disturbing in areas with considerable political implications. Critics point to the 1990 example when the discovery of a putative gene for ''alcoholism'' (the D2, or dopamine receptor locus, on chromosome 11) was announced with great fanfare as front-page news in many newspapers and as cover stories for a number of magazines. Failure of other researchers to replicate the study has resulted in the ultimate discrediting of the work; announcement of this failure got no publicity at all. Critics argue that those who do work in the area of human behavior genetics should use more than the usual amount of caution in announcing any purported discoveries in the press or in other arenas.

A particularly egregious aspect of publicizing results in the press that has been troublesome to behavioral biologists occurred in early 2000 with a flurry of prepub-lication attention given to a book claiming that there is an evolutionary basis for rape. In A Natural History of Rape: Biological Bases of Sexual Coercion, Randy Thornhill, who studies insect mating behaviors, Craig Palmer, an anthropologist, and Margo Wilson, argue that rape is evolutionarily advantageous for males and has been selected over the last million or more years as a way of maximizing the transmission of a male's genes to the next generation. Coming with a full set of instructions that included admonitions to women that they should not dress ''provocatively'' if they want to avoid being raped, the book was strongly criticized by geneticists and evolutionary biologists as representing the worst of excesses in human behavioral genetics. With little empirical evidence to back up such claims, evolutionary models have repeatedly been fabricated from conjecture, another version of what evolutionists referred to in the 1970s as ''just-so-stories'' (after Rudyard Kiplings's famous book of the same title in which, among other essays, was a famous discourse on ''How the Elephant Got His Trunk''). Evolutionary biologists have criticized ''just-so-stories'' for being imaginative scenarios supported by no evidence other than that they sound possible. In the case of A Natural History of Rape, ethicists argue that widespread belief in such ideas, especially by young males, could lead to an increase in the incidence of rape on the grounds that young men were only ''doing what comes naturally.'' The further prospect that the biological argument might be used in court to exonerate or gain a lighter penalty for perpetrators of rape is also seen as raising legal as well as moral and ethical questions.

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