Behavioral Genetics And Education Research Paper

1. Concepts

1.1 Quantitative Behavioral Genetics

Quantitative behavioral genetics accounts for individual differences in behavior in terms of several sources of variability, and it is applied to mental disorders as well as to continuously distributed traits such as IQ that are influenced by many genes. This approach is usually traced back to Galton (1869), who was interested in the origins of excellence. Modern quantitative behavioral genetics partitions the observed or phenotypic variance (the mean squared deviation of individual scores from the sample mean) into (a) additive genetic variance or the additive effects of single genes, (b) gene dominance or the interactive effects of the two homologous genes at the same gene locus, (c) epistasis or the interactive effects of genes at different loci, (d) shared environment, defined as contributing to the similarity of persons reared in the same family independent of their biological related- ness, and (e) nonshared environment, defined as not contributing to the similarity of persons reared in the same family. In this context, environment refers to the effects of all nongenetic sources of variance, including nongenetic biological variables such as infectious diseases and nutrition.

The importance of these sources of variance is inferred from the correlations between relatives, taking their genetic and environmental similarities into account. For example, the correlation between monozygotic (MZ) twins reared apart estimates the combined influence of all genetic sources of variance (broad-sense heritability), whereas twice the correlation between a biological mother and her adopted-away offspring estimates the importance of additive genetic variance (narrow-sense heritability) only. Moreover, the reliable differences between MZ co-twins reared together estimate the importance of the nonshared environment, and the correlation between adoptive siblings estimates the importance of the shared environment. Correlations between different kinds of relatives may be analyzed simultaneously, using structural equation modeling (Neale and Cardon 1992).

1.2 Heritability

An important concept in quantitative behavioral genetics is the heritability (h ) of a trait, defined as the ratio of the genetic variance (V_G) to the phenotypic variance (V_P):

For example, if the correlation between the trait levels of MZ twins reared apart is r =0.50, it is concluded that 50 percent of the individual differences in that trait reflect effects of different genes, whereas the other 50 percent reflect effects of nonshared environments, imp lying that the broad-sense heritability of that trait is h²=0.50. Note that correlations between MZ cotwins depend on the genetic variability between twin pairs and on the variability of their environments, implying that the heritability statistic may differ between populations and change across time. For example, if impoverished family environments are improved by education to foster equal opportunities for all children, the average level of performance will rise. But as the previous variability in environments will thereby be reduced, the heritability estimates will also rise. Both seem to have happened in industrialized countries in the latter part of the twentieth century (Flynn 1987, Plomin et al. 1997a).

1.3 Mean Levels Versus Individual Differences

Hence influences on mean levels have to be distinguished from influences on individual differences. Heritability may be high because the environmental conditions in that population are homogeneous, leaving genetic factors to account for the individual differences. That, however, does not imply that the mean level cannot be raised by environmental intervention. Rather, by improving environmental conditions for entire populations, mean levels may be raised in spite of high heritabilities of the individual differences. This is not purely hypothetical: Although IQ is substantially heritable within cohorts, massive gains across cohorts in average IQ have been observed in industrialized countries since World War II (Flynn 1987), reflecting secular changes in environmental conditions. Another example is stature: although being highly heritable within cohorts, average height increased substantially during the twentieth century, probably owing to secular trends in nutrition (Vogel and Motulsky 1997).

1.4 Developmental Behavioral Genetics

This is the study of genetic and environmental influences on individual differences in behavioral development, and it combines a behavior–genetic with a developmental approach. A topic for developmental behavioral genetics that can be studied longitudinally as well as cross-sectionally is how heritability estimates are related to the age of the samples under study.

1.5 Genetic And Environmental Continuity And Change

These concepts refer to the extent that observed stability and change in individual differences reflect stability and change of the effects of genes and environments. As they are inferred from the pattern of concurrent correlations and longitudinal cross correlations between relatives, they require longitudinal studies. These allow for estimates of (a) genetic and environmental influences at time 1, (b) genetic and environmental influences at time 2, and (c) the genetic and environmental continuities between time 1 and time 2. For example, if the concurrent correlations between MZ twins reared apart are higher than their longitudinal cross-correlations (twin A at time 1× twin B at time 2, and vice versa), this shows lack of continuity of the effects of genes. Correspondingly, if the concurrent correlations between adoptive siblings are higher than their longitudinal cross-correlations, this shows lack of continuity of the effects of the shared environment.

If heritabilities are high but genetic continuities are low, genes are important but their effects change with age. For example, genes that foster high abilities in childhood may have less beneficial effects in adulthood. It is important to realize that, although persons’ DNA usually does not change, the effects of their genes may change across time, hence genetic influence does not imply lack of change. This is obvious for the genetically driven physical changes during puberty, but it has also been shown for individual differences in cognitive development that are more strongly synchronized for monozygotic than for dizygotic (DZ) twins (Wilson 1983).

2. Findings

A meta-analysis of the world literature on family resemblance in IQ by Loehlin (1989) yielded broadsense heritability estimates of about 0.50 for IQ. Moreover, it suggested moderate effects of the shared environment. That genes have an effect on individual differences in IQ is meanwhile established beyond any reasonable doubt. The evidence concerning genetic and environmental influences on personality traits such as extraversion and conscientiousness also suggests substantial heritabilities (Loehlin 1992). The mechanisms that mediate the effects of genes on behavior are largely unknown, however.

2.1 Heritability Estimates And Age

Loehlin’s (1989) estimates for IQ were inferred from a meta-analysis of diverse studies that tested participants at various ages, whereas more fine-grained analyses show that heritability estimates depend on age: In a meta-analysis of 103 cross-sectional twin studies, McCartney et al. (1990) focused on relations between twin age and twin similarity. Whereas older MZ twins were more similar in IQ than younger MZ twins, the opposite was found for DZ twins. This suggests that genes become more influential whereas shared environment becomes less influential across the life span.

This finding is confirmed by longitudinal twin and adoption studies: The Louisville Twin Study followed the cognitive development of MZ and DZ twins from 3 months to 15 years. Whereas the correlations between MZ twins increased from r=0.66 to r=0.88, the correlations between DZ twins decreased from r=0.67 to r=0.54, suggesting growing genetic and diminishing environmental influence from infancy to adolescence (Wilson 1983). In the longitudinal Colorado Adoption Project (Plomin et al. 1997b), the IQ correlation between adoptive parents and their adopted children dropped from r=0.20 at age 3 years to r =0.00 in adolescence, suggesting weak and diminishing shared environmental influence. In contrast, the IQ correlation between biological parents and their own children in control families rose from 0.17 at age 3 years to 0.30 at age 16 years, suggesting narrow-sense heritabilities of 0.60 in adolescence.

2.2 Gene–Environment Covariation

The most reasonable explanation of the growing genetic influence on cognitive abilities across the life span is that the environments that contribute to individual differences in cognitive development become increasingly a function of the individuals’ genotypes (Plomin 1994). The older children and adolescents become, the more they create and choose their environments, and they probably do so in ways such that their environments fit their genetically influenced dispositions. For example, more intelligent children tend to be more successful in intellectually demanding tasks and thus tend to persist with them, whereas less intelligent children experience more failure in those tasks and therefore turn to less demanding activities. This is sometimes referred to as a smorgasbord view of the environment: The technical term for that is genotype–environment covariation of the evoked (or reactive) and active variety.

3. Implications For Education

The main implication of developmental behavioral genetics for education is that there are ubiquitous genetically influenced individual differences, as has been shown most convincingly for intelligence. General intelligence is problem-solving ability, and it is strongly related to speed and potential of learning. Thus individual differences in learning speed and learning potential are also genetically influenced. Indeed, the dynamic testing approach (Grigorenko and Sternberg 1998) focuses directly on the potential to learn.

However, environments are also important. Behavior–genetic research shows that at least some of the variability in behavioral traits reflects variability in environments, and this is only the variability between environments in a particular population at a given time. Most behavioral–genetic research is insensitive to secular changes in environmental conditions that affect entire populations in a similar way as is likely to have occurred for IQ (Flynn 1987). And it is also insensitive to potential environmental changes that might be induced by purposeful intervention. The latter is the subject of educational, not behavioral– genetic research.

Hence the prime message from behavioral genetics for education is that the uniqueness of human personality and abilities requires different or even individualized intervention programs. It is unlikely that there is a curriculum that fits all children’s needs despite their different abilities and interests. Neither can all children be expected to attain the same level of proficiency, nor can they be expected to learn at the same pace, nor can a given child be expected to attain the same level of expertise in all fields. That, of course, makes education not less important but more demanding.

Moreover, there are two important lessons to be learnt from the principle of gene–environment covariation. First, if environmental variables are associated with behavior, we do not know whether they are the cause of that behavior. The sources of the behavior may as well be genetic, the genes being associated with the environmental variables (Plomin 1994). It needs genetically informative designs such as twin and adoption studies to disentangle these influences. Second, the principle of gene–environment covariation suggests that children should be provided with a variety of microenvironmental contexts to select their preferred niches that fit their inherited dispositions (Wachs 1992). The best learning environment allows children to discover and develop their natural interests, proclivities, and special talents. Behavior– genetic findings indicate that they will do so anyway, and it is probably useful to help them to proceed on their way.

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