Francis Galton’s publication on “hereditary genius” in 1869 was one of the earliest studies on the role of heredity with respect to particular traits and abilities in individuals based on familial relationships. Since then, individual differences have been studied by all models and theories of development. Gesell (1954) regarded individual differences as hereditary factors that controlled the rate of development and were immune to environmental experience. Like Gesell, Piaget and his followers consider individual differences in the rate of development as a consequence of hereditary factors. Behaviorists or environmentalists dominated the field from the 1930s to the early 1960s, and they placed a strong emphasis on environmental controls as opposed to hereditary influences. They attributed individual differences to the different cumulative histories of individuals. The Freudian model of development perceives individual differences as a variable affecting the interplay of the organism and the environment. Presently, with more evidence being gathered supporting genetic contributions to behavior, a more balanced view has developed. Plomin (1988), recognizing this, had commented: “As the pendulum swings from environmentalism, it is important that the pendulum be caught mid-swing before its momentum carries it to biological determinism.” (p.110).
Evidence from behavioral genetics suggests that heritable and nonheritable characteristics are both important for individual differences. Behavior geneticists have worked on estimating the degree to which genetic and environmental factors and their interaction account for variability in the population in phenotypic characteristics. In so doing, scientists often explore heritability, the proportion of phenotypic variance accounted for by genetic differences among individuals. In particular, heritability estimates indicate the proportion of observed variance in individual difference scores that can be attributed to genetic factors in the particular population studied. But they do not indicate the degree to which a given trait reflects the influence of heredity for an individual, nor do they say anything about whether the expression of the trait may change in response to the environmental changes or why a particular individual does or does not have a given trait. Thus, in the strictest sense, behavioral genetics that is based on heritability is more the study of group differences than of individual differences, even though the groups are constituted by individuals who differ in comparison to one another on an individual-difference dimension. According to Caprara and Cervone (163), whatever the heritability of a phenotype, environmental considerations may still be crucial to understanding it. For example, although sociopolitical attitudes may be heritable, political views are shaped only in a sociopolitical environment.
The heritability of individual differences in behavior may be assessed by several methods (Plomin, DeFries, & McClearn 105). For example, selective breeding studies of animals may be undertaken, using cross-fostering to control for upbringing. However, it is not possible to manipulate human genetic material through such methods as used in the case of animals. Instead, scientists use experiments of nature and of culture (Rushton et al. 1192). Twin designs compare individuals whose genetic relatedness varies systematically and adoption designs distinguish genetic and environmental sources of variability by comparing biological relatives raised apart with unrelated individuals raised together. Family designs assess the resemblance between family members as a function of their relatedness (Rushton et al. 1195).
The relation between gene action and behavioral development has been well summarized by Thiessen (1972, p. 87): “Gene influence in behavior is always indirect. Hence the regulatory processes of a behavior can be assigned to structural and physiological consequences of gene action and developmental canalization. The blueprint for behavior may be a heritable characteristic of DNA, but its ultimate architecture is a problem for biochemistry and physiology”. It has been hypothesized that several kinds of genes exist: structural genes to specify the proteins to be synthesized, operator genes to turn protein synthesis on and off in adjacent structural genes, and regulator genes to repress or activate the operator and structural genes in a larger system. Regulatory genes are probably the ones responsible for species and individual differentiation through control of the expression of structural genes. Thiessen (1972) says that the diversity of individuals and species can be explained by the regulatory genes that have the capability to modify the expression of basic biochemical processes. In other words, “the greatest proportion of phenotypic variance, at least in mammalian species, is probably due to regulatory rather than structural genes–genes that activate, deactivate, or otherwise alter the expression of a finite number of structural genes” (Thiessen, 1972, p. 124 ).
Despite advances in studying the link between heritability and individual differences, there are some limitations in these studies. For example, the studies are based on the assumption that everyone can be analyzed using the same model, and therefore a single population-wide estimate of heritability is computed. The approach does not include the possibility that a particular train may be determined primarily by genetics for some individuals but by environmental factors for others (Caprara and Cervone, 165). Eley, Lichtenstein, and Stevenson (1999) have studied two large independent samples from Sweden and Britain and explored the issue of the heritability of antisocial behavior. It was found that genetic factors strongly accounted for individual differences in antisocial behavior among women, whereas shared environmental factors were a major determinant among men (Eley et al., 155). The authors explain that this time is due to the fact that men’s level of exposure to environmental factors that promote non-aggressive antisocial behavior is greater than women’s. The idea that different subsets of the population inherit particular temperament profiles implies the possibility that the heritability of a given characteristic varies from group to group. Caprara and Cervone hold that among subsets of people who inherit inhibited and uninhibited profiles, genetic factors may account for individual differences, whereas among all others, these differences may be due to the environment. There is a broad agreement that there is a heritability of over 70% for general cognitive ability or IQ (McGue, Bouchard, Iacono, and Lykken 60). The average heritability for each of the Big Five Personality dimensions is 50% when measuring through self-reports or observer ratings (Loehlin 102).
Additive gene effects refer to independent effects of genes that add up to influence a trait, such as height and weight. Nonadditive genetic effects refer to the influences caused by the interaction of genes. Resemblances between identical twins reflect both additive and nonadditive effects, whereas resemblances between other genetically related individuals reflect mostly nonadditive effects. Lykken, McGue, Tellegn, and Bouchard (1992) have suggested that nonadditive genetic factors play an important role in the development of personality as personality traits may be an “emergent property of a configuration of more basic traits that are themselves genetic in origin” (p. 1569). This phenomenon is called emergence. Emergency traits are traits such as extraversion, ego control, leadership, and creativity and are produced by the interaction between polymorphic genes and by the reaction of polymorphic genes with the environment “both embryologically and during subsequent development” (Lykken et al., 1992, p. 1575). Childhood temperamental dimensions such as negative affectively activity level and emotional regulation are also found to have considerable heritability (Caprara and Cervone 163).
Genetic effects vary with age, and though one might expect genetic influences to diminish with age due to the accumulation of environmental experiences, it has been found that genetic factors account for greater variance in IQ as age increases (Caprara and Cervone 164). But all other studies on personality dimensions show that with increasing age, genetics account for less variation in personality, and environment accounts for more. Twin studies based on 15000 twins, conducted over a six-year time span, showed that heritability coefficients for both extroversion and neuroticism decreased with age (Caprara and Cervone 164). This shows that environmental influences dominate in old age on personality dimensions. McGue, Bacon, and Lykken suggest that personality stability is the outcome of genetic factors, whereas personality change occurs as a result of environmental influences (McGue et al. 109).
Although physical traits such as height and weight are strongly genetically determined, it is the environment that decides the way to treat tall and short people, fat and thin people, etc. A new direction for further research on heritability and individual differences is to examine environmental factors that are essential to the link from genes to behavioral expressions and social outcomes. Molecular genetics is a new field that tries to explain the processes leading from genes to action, recognizing that no single gene is likely to control a personality phenotype.
Works Cited
Caprara, V. Gian and Cervone, Daniel (2000). Personality: Determinants, Dynamics, and Potentials. Cambridge University Press. 2000.
Eley, T.C; Linchenstein, P. and Stevenson, J. (1930). Sex differences in the etiology of aggressive and nonaggressive antisocial behavior: Results from two twin studies. Child development 1999. Vol. 70, No. 1, p. 155-168.
Loehlin, J.C. (1992). Genes and Environment in Personality Development. Sage Publications. Newbury Park. California.
Lykken, D.T., McGuie M., Tellegen, A. and Bouchard T. J. (1992). Emergenisis. American Psychologists, 47, 1565-77.
McGue M.; Bouchard T.J.; Iacono W.G.; Lykken D.T. (1993) Behavioral genetics of cognitive ability: a life-span perspective. American Psychological Association, Washington, DC, pp 59–76.
McGue, Matt; Bacon, Steven and Lykken, T. David (1995). Personality Stability and Change in Early Adulthood: A Behavioral Genetic Analysis. Developmental Psychology 1993. Vol. 29, No. 1, 96-109.
Plomin, R., DeFries, J. C., McClearn, G. E. (1980). Behavioral genetics: A primer. Freeman Publishers. San Francisco.
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