INTELLIGENCE: HEREDITY OR ENVIRONMENT (OR BOTH?) Is intelligence given by nature or nature? Was Galton closer to being correct in thinking it was hereditary (nature)? Or was Binet and the opposite view (nurture) more correct? → the nature/nurture debate is ongoing and has serious sociopolitical implications

HEREDITARY INFLUENCE Due to being raised in similar environments, studies of identical (same genes) and fraternal (similar genes) twins offer relevant evidence → fraternal twins have high test-score correlations (.60), but not as high as identical twins (.86), suggesting IQ is inherited

HEREDITARY INFLUENCE → identical twins raised separately still have a higher IQ correlation than fraternal twins

HEREDITARY INFLUENCE Adopted children show higher correlation with their biological parents (who they were not raised by) than with their adoptive parents (who they were) → their correlation with their adoptive siblings is higher than either set of parents however

HEREDITARY INFLUENCE → as adoptees become adults the correlation decreases between them and their adoptive family to close to zero (and increases for identical twins)

HEREDITARY INFLUENCE Intelligence heritability – the proportion of variation among individuals that we can attribute to genes – estimates range widely from as high as 80% down to 40% → in other words, environmental factors only account for 20% of intelligence according to the first figure, or as much as 60% in the 2nd → despite consensus estimates around 50%, heritability is a generalized, variable, and unfixed estimate

ENVIRONMENTAL INFLUENCE Despite correlation studies showing the influence of genetics on intelligence, they also point to environmental influence → identical twins, biological siblings, and adoptive siblings* raised together all show higher correlations than those raised apart * Compared w/bio. siblings raised apart

ENVIRONMENTAL INFLUENCE Studies of children in destitute orphanages (Iran/Romania) and impoverished regions (Appalachia) show declines in IQ as they age → malnutrition, social isolation, sensory deprivation and/or poor teachers can all contribute to slowing brain development

ENVIRONMENTAL INFLUENCE → additional studies have also shown that children removed from these environments and placed in favorable circumstances will outpace their siblings or peers left behind

ENVIRONMENTAL INFLUENCE The so-called Flynn effect – comparative IQ scores increased throughout the 20th century (it was harder to get a 100 in 1990 than in 1940) – suggests the effect of such environmental factors as nutrition and schooling on intelligence

HEREDITY AND ENVIRONMENT Sandra Scarr posits that we are born with upper and lower limits of IQ (our reaction range) and the environment we are in determines where we fall within those limits → being raised in a high-quality environment will allow an individual to score near the top of their range and vice versa

HEREDITY AND ENVIRONMENT Identifying specific genes for intelligence could help isolate genetic and environmental factors, but genes so far discovered to be related to intelligence each account for less than ½ of 1% of intelligence variation

CULTURE AND IQ Racial/ethnic/cultural group differences in IQ are an agreed-upon-fact (despite a closing of the gap in recent years in some cases), what is not agreed upon is the reason → with high IQs leading to higher educational opportunities and thus better job opportunities, the debate has profound implications

CULTURE AND IQ One side of debate represented by Arthur Jensen and the book The Bell Curve (‘94) says that IQ differences between groups ARE largely based on genetics → this side points to heritability of intelligence estimates at 80% as proof

CULTURE AND IQ → critics argue that high heritability of intelligence may explain within-group differences, but between-group differences can still be based on environmental factors

CULTURE AND IQ → once again, the Flynn effect points to environmental factors as highly relevant to between-group IQ variation

CULTURE AND IQ The other side of the debate argues that socioeconomic factors more than anything explain IQ variation within groups → minority groups are overrepresented in the lower classes, but even taking race out of the picture, lower class children score on average much lower

CULTURE AND IQ Claude Steele has argued that another source of group IQ variation is stereotype threat: the fear that a poor test performance will confirm negative stereotypes of a group → the cognitive resources necessary for test success are usurped (used up) by anxiety and self-doubt