Intelligence, Heritability and Race

Majority of notes I took from the fantastic Maltby, Day and Macaskill ‘Individual Differences’ textbook (click image)

 


 

SECTION A – INTELLIGENCE: THE NATURE VS NURTURE DEBATE

SIR FRANCIS GALTON (1869)

– children inherit their intelligence from their parents (just as Darwin said with physical features)

– the first to distinguish & coin terms nature and nurture: considered environmental influence

– conceived of twin and adoption studies to determine nature/nurture

 

WHAT DO WE MEAN BY HERITABILITY OF INTELLIGENCE?

Genetic heritability: the extent to which any phenotype (outward feature – appearance, behaviour, intelligence) is passed from parents to kids due to genes, on average, across population

– e.g. one’s nose: 100% shared variance, env 0 variability; intelligence: lower shared variance & higher heritable variability; football team: 0 shared variance; env high variability

h2 (squared): estimated average of genetic heritability. Is subtracted from 100% to calc env contribution

 

HERITABILITY ESTIMATES OF INTELLIGENCE

– family, twin & adoption studies look at similarities & diffs bet. populations to assess influence of genes on intelligence

Ridley (1999) compiled these various studies to find the concordance rate for IQ

– e.g. concordance rate (same score of 2 people) for identical twins reared apart = 76% (more…)

 

CONSIDERATIONS WITHIN BEHAVIOURAL GENETICS & INTELLIGENCE

e.g. Plomin (2004) questions the additive principle of determining heritability of intelligence

1) estimating env is 100% – heritability: no direct measure of env, & heritability prone to error

2) better to see env as triggering certain genetic behaviours: g x e (not g + e)

move to g x e influenced by factors in the next 4 sections:

 

CONCEPTIONS OF GENETIC HERITABILITY & THE ENVIRONMENT

Carey (2002) if genetic heritability estimated at 60% for the population, doesn’t mean that for me 60% is g and 40% env – g is an average

 

DIFFERENT TYPES OF GENETIC VARIANCE

Bouchard & McGue (1981) – genes not a single entity: they interact and suppress each other

– original estimates of genes inherited or shared by siblings oversimplified

 

PROBLEMS WITH REPRESENTATIVENESS OF TWIN & ADOPTION STUDIES

Kamin & Goldberger (2002): twin studies overestimate genetic role as mz twins have more similar envs than dz twins

Stoolmiller (1998): adoption agencies may place kids with high income families – this env variable not explored

 

ASSORTATIVE MATING

– people often mate with similar people, but generic variation theory assumes chance genetic similarity, which isn’t true

 

MODERN ESTIMATES OF THE GENETIC HERITABILITY OF INTELLIGENCE

– American psychological Assoc task force (headed by Neisser et al 1996) estimates heritability ranging from 40-80%

– Chipeur et al. (1990) suggested 50% genetic heritability of intelligence, which is a commonly accepted viewpoint

 


 

ENVIRONMENTAL INFLUENCES ON INTELLIGENCE:

BIOLOGICAL VARIABLES AND MATERNAL EFFECTS

NUTRITION

Oddy et al (2004) – 2000 Australian 0-8yrs; linked duration of breastfeeding to intelligence(IQ)

Lynn (1990) – nutrition is part of a total nurturing environment >> increased intelligence

 

LEAD

McMichael et al (1986) – people living near leadworks: early exposure affects intelligence (IQ) through childhood (other factors controlled)

 

PRENATAL FACTORS

Mortensen et al (2005) – 3044 males 18-19yrs: maternal smokers >> lower IQ (other factors controlled)

Neisser et al (1996) – smoking & alcohol consumption in preganancy affects intelligence

 

FAMILY ENVIRONMENT

SHARED AND NON-SHARED ENVIRONMENTS

Baker & Daniels (1990): degree of different experiences in childhood bet. siblings >> intelligence differences in adulthood

 

WITHIN-FAMILY FACTORS

Harris (1995) – e.g. twins with different intelligence might be encouraged to be similar e.g. less nerdy or more studious

– so family situation shapes intelligence & can lead to overestimation of heritability

 

OUTSIDE FAMILY FACTORS

Harris (1995) – social groups that we identify with influence our intelligence

– behavioural geneticists may oversimply family influence & overestimate heretability effects

 

SOCIOECONOMIC STATUS OF THE FAMILY

Jensen (1993) – socioeconomic status correlated with IQ r=.3/.4; with 45 points bet. Class I& V

Mackintosh (1998) – followed 17000 UK babies for 33 years; found link even accounting for other factors; also 10pt IQ difference bet. Class I & V

Wahlsten (1997) – IQ improved by 16 points when child moved from low to high socioeconomic family (adoption)

 

BIRTH ORDER, FAMILY SIZE AND INTELLIGENCE

Belmont & Marolla (1973) – intelligence decreases with family size and birth position

 

FAMILY SIZE & INTELLIGENCE

BUT Rodgers et al (2000) – found no relationship with family size; Belmont study probably had confounds

 

BIRTH ORDER & INTELLIGENCE

– research supports Belmont & Marolla’s research but various hypotheses explaining why

Rodgers (2001) – admix hyp: people with high IQ have less kids; acts as a confound

Downey (2001) – resource dilution hyp: time etc available for each kids falls as family grows

Zajonc (1976) – confluence model: older kids explain things to younger; kids hear more kid talk as family grows

 

EDUCATION & INTELLIGENCE

Sorel & Cohen (1989) – length of schooling predicted performance on verbal tests

Ceci (1991) – R – regular attendance and more schooling >> higher IQ scores

Wahlsten (1997) – delays in starting school causes IQ scores to drop by 5pts a year

McKey et al (1995) – operation Head Start kids had immediate (short-term) gains in IQ

Zill et al (1995) – poverty means parents can’t give kids head start for school (e.g. nutrition, reading skills)

 

FINAL COMMENTS ON GENETIC & ENV INFLUENCES ON INTELLIGENCE

Bouchard & Loehlin (2001) – present questions for thinking about any genetic vs env debate (useful for an advanced approach to the question?) e.g. what type of gene is involved

 


 

SECTION B – THE BELL CURVE: RACE DIFFERENCES IN INTELLIGENCE

Herrnstein & Murray (HM) 1994 – THE BELL CURVE: INTELLIGENCE & CLASS STRUCTURE

– (1) there’s a general factor of intelligence and people differ in it; (2) IQ tests measure it best

– (3) IQ tests measure intelligence, (5) they are not biased against particular groups

– (6) intelligence is 40-80% heritable and IQ scores are stable

 

H&M AND THEIR ‘COGNITIVE ELITE’

– the most important factor in university admission was intelligence, not class or wealth

– intelligence predicts job performance; these 2 factors are behind emergence of cognitive elite

 

H&M, IQ SCORES & SOCIO ECONOMIC PROBLEMS – THE LOWER END OF THE BELL

– intelligence clearly underlies e.g. unemployment, criminality, school dropout rate

 

H&M, THE RELATIONSHIP BET. RACE & IQ: IMPLICATIONS FOR SOCIAL POLICY

– white Americans scored 15-18 IQ points higher that black

– women with lower IQ have more kids, so lots of kids being born in low intelligence env

– not surprising that attempts to raise intelligence have failed as it’s 40-80% inherited

 

CRITICISMS OF THE BELL CURVE: INTELLIGENCE & CLASS STRUCTURE IN US LIFE

(numbers in brackets here correspond with numbered HM claims above)

(1) not a single g: hierarchy is better understood as a hierarchy of abilities e.g. Carroll; or intelligence has 2 components (crystallised and fluid intelligence) e.g. Cattell

(2) given (1) it’s debatable that there’s a test that can measure these different models

(3) immigrants might come from countries where different types of intelligence are emphasized

(3) Sternberg (1985) – there are different conceptions of intelligence across diff. Professions

(4) lots of support e.g. Deary et al (2000) r=0.63+ over 60 years; BUT see Ceci, Wahlsten, nutrition above

(5) Support from Neisser et al (1996) BUT questionable in that the tests measure things valued in the West e.g. speed, info sorting etc

(6) Support from Neisser et al (though note that 40-80% inheritance is quite large)

(6) biologists say making distinctions bet. people based on skin colour/race doesn’t make sense, so using it to account for differences in IQ is questionable

Graves (2001): genetic variance within a population is 10 times greater than variance between races; 2 people of same race might vary more genetically, than 2 people of different races

 

STATISTICAL & EVIDENCE BASED PROBLEMS IN THE BELL CURVE ARGUMENTS

Kamin (1995): forgot correlation is not causation e.g. IQ & social status; also points out that some of the studies H&M cited were dodgy