Defining Intelligence: Genetic V.S. Environmental Influences


By Dawn Boykin

General cognitive ability is one of the most well studied domains in behavioral genetics. Nearly all of this genetic research is based on a model in which cognitive abilities are organized hierarchically , from specific test to broad factors to general cognitive abilities(often called g); and the relative influences of nature and nurture on G have been studied since the beginning of behavioral genetics. There are hundreds of tests of diverse cognitive abilities. These tests measure several broad factors such as verbal ability, spatial ability, memory, and speed of processing. Such tests are widely used in schools, industry, and the military and clinical practice. Although few critics remain, g is widely accepted as a valued concept by experts. It is less clear what g is, whether g is due to a single general process such as executive functioning or speed of information processing, or whether it represents a concentration of more specific cognitive processes.

The idea of genetic contribution to g has produced controversy in the media, especially following the 1994 publication of The Bell Curve by Bernstein and Murray (1994). In fact, these authors scarcely touched on genetics and did not view genetic evidence as crucial to their argument. Despite this controversy, there is considerable consensus among scientists – even those who aren’t geneticists that g is substantially heritable.  However, heritability estimates are also said to be affected by such factors as assortive mating (which is substantial for general cognitive ability) and non additive genetic variance (dominance and at the epitasis).

The phrase general cognitive ability to describe g is preferable to the word intelligence because the latter has so many different meanings in psychology and in general language. Clearly there is more to cognition then g. Despite the massive data pointing to the reality of g, considerable controversy continues to surround g and intelligence tests, especially in the media. There is a wide gap between what laypeople (including scientists in other fields) believe and what experts believe. In addition, just as there is more to cognition than g, there is clearly more to achievement than cognition. Personality, motivation, and creativity all play a part in how well someone does in life.

Highlights in the history of human research on genetics and g include two early adoption studies which found that IQ correlations were greater in non-adoptive then in adoptive families, suggesting a genetic influence . The first adoption study that included IQ data for biological parents of adopted away offspring’s also showed significant parent – offspring correlation, again suggesting genetic influence. However, beginning in the early 1960s, the Louisville twin study was the first major longitudinal twin study of IQ that charted the developmental course of genetic and environmental influences.

During the 1960s environmentalism, which had been rampant until then in American psychology, began to wane, and the stage was set for increased acceptance of genetic influence on g. In 1981, a review of genetic research on g was published that summarize results from dozens of studies which is based on average IQ correlations for family, adoption and twin study designs.

 Family, twin and adoption studies converge on the conclusion that about half of the total variance of measures of general cognitive ability can be accounted for by genetic factors. Heritability estimates are affected by sort of meeting and not additive genetic variance and about half of the environmental variance for g appears to be accounted for by shared environmental factors. First-degree relatives living together are moderately correlated for g (about .45), this resemblance could be due to genetics or to environmental influences, because such relative share both.

On the other hand, adoption designs disentangle these genetic and environmental sources of resemblance. This is because adopted- apart parents and offspring’s and siblings share heritability but not family environment, their similarity indicates that resemblance among family members is due in part by genetic factors. For g, the correlation between adopted children and their genetic parent is .24. The correlation between genetically related siblings reared apart is also .24. Because first-degree relatives only 50% similar genetically, doubling these correlations gives a rough estimate of heritability of 48%. The 20 method supports this conclusion heritability of g as well. Identical twins are nearly as the same person tested twice (test – retest correlations for g are generally between .80 and .90) the average twin correlation or .86 for identical twins and .64 fraternal twins.

If half of the variance of g can be accounted for by heredity, the other half is attributed to environmental (plus errors of measurement). Some of this environmental influence appears to be shared by family members, making them similar to one another. Estimates of the importance of shared environmental influence come from correlations for adoptive parent and children and for adoptive siblings; with an impressive correlation of .32 for adoptive siblings. Because they are unrelated genetically, what makes adoptive siblings similar is shared rearing having the same parents, the same diet, attending the same schools, etc. the correlation for adoptive parents and their adoptive children is lower(r=.19) than that for adoptive siblings, suggesting that shared environment accounts for less resemblance between parents and offspring’s than between siblings.

Assortive mating is among several other factors which needs to be considered for a more refined estimate of genetic influence, which refers to nonrandom mating; In a sense that individuals who may tend to be similar. Assortive mating for g is substantial with average spouse correlations of about .40. And part, spouses select each other for g on the basis of education. Spouses correlate about .6o for education, which correlates about .60 with g. Assorted mating increases genetic variance in the population and it affects estimates of heritability alike.

Non-additive genetic variance also affects heritability estimates. Additive genetics effects occur when alleles at a locus and across loci “add up” to affect behavior. However, sometimes the effects of alleles can be different in the present of other alleles. This interactive affects the term non additive. Dominance is a non additive genetic effect in which alleles at a locus interact rather than add up. When several genes affect the behavior the allele that different Loki can add up to a fate behavior are they can interact. This type of interaction is called epitasis.

Additive genetic variance is what makes us resemble our parents. We resemble our parents to the extent that each allele that we share with our parents has an average additive effect. Because we don’t have exactly the same combination of alleles is our parents (we inherent only one of each of their pairs of alleles), we will differ from our parents for non additive interactions as a result of dominance or epitasis. For g, the correlations suggest that genetic influences largely additive. The presence of dominance can be seen from studies of inbreeding. Inbreeding data suggest some dominance for g, because inbreeding lowers IQ.

The evidence for a strong genetic contribution to general cognitive ability (g) is clearer than for any other area of psychology. Although g has been central in the nature versus nurture debate, few scientists now seriously dispute the conclusion that general cognitive ability show significant genetic influence. However, the magnitude of genetic influence is still not universally appreciated. Take together; the extensive body of research suggests that about half of the total variance of measures of g can be accounted for by genetic factors. The heritability of g increases during individual’s lifespan, and the influence of shared environment diminishes sharply after adolescence. Longitudinal genetic analysis of g suggest that genetic factors primarily contribute to continuity, although some evidence for genetic change has been found, as in the transition from early to middle childhood. An attempt to identify some of the genes responsible for the heritability of g is underway, including candidate gene studies, QTL linkage and genome wide association studies.

The Bell Curve by Bernstein and Murray (1994)

Behavioral Genetics, 5th edition, Plomin, Defries, McClearn, McGuffin (2008,2001,1997,1990,1980)


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