maltby chapter 13 & 14
francis galton 1822-1911
Polymath
ā¢ Created the first weather maps
ā¢ Designed fingerprint classification systems
ā¢ Invented correlation
ā¢ Invented regression
ā¢ First to use the survey as a form of data collection
ā¢ Forefather of individual differences
ā¢ Measurement and quantification
ā¢ First to attempt to measure intelligence directly
ā¢ Reaction time, sight and hearing, ability to distinguish colour
ā¢ Coined ānature versus nurtureā
ā¢ Coined āeugenicsā
Galton. (1869). Hereditary Genuis.?
Theorised that intelligence ran in families
Galton and eugenics
1885: Outlined genetic principles of intelligence
ā¢ Proposed that intelligence was inherited through genes
ā¢ Thought that people with lower intelligence were having more
children, causing the human race to become weaker
ā¢ Proposed that white people had higher levels of intelligence
than Black people and other ethnic groups
ā¢ Proposed using artificial selection to increase intelligence
among humans
ā¢ Protecting the āweakā was at odds with natural selection
eugenics
Theory that humans can be improved by selective breeding
ā¢ Based on incorrect understanding of genetics
ā¢ Believed complex human traits such as intelligence were inherited in a simple
fashion from parent to child (like Huntingtonās disease)
ā¢ Believed complex human traits were solely the outcome of genetic inheritance (not influenced by environment)
ā¢ āPositiveā eugenics: encouraging increased reproduction in people with āsuperiorā traits
ā¢ Encourage white people from higher social classes to have more children
ā¢ āNegativeā eugenics: discouraging or stopping reproduction in people with āundesirableā traits
ā¢ Encourage people from low social classes to not have children
ā¢ Used pseudoscientific theories to argue for the superiority of white Europeans
eugenics in the US
Lewis Terman, president of the American Psychology
Association (1923)
ā¢ Low intelligence often found in Spanish-Indian, Black, and Mexican
families
ā¢ This low intelligence was inherited
ā¢ These groups should be segregated
ā¢ Concern that these groups had more children than white Americans did.
sterilisation laws in the US
922: Model Eugenical Sterilisation Law: Mandatory sterilisation
for certain groups
ā¢ āFeeble-mindedā (low intelligence)
ā¢ The āinsaneā
ā¢ The criminal
ā¢ The epileptic
ā¢ The blind
ā¢ The deformed and dependent (orphans, homeless)
ā¢ Estimated that >42,000 people were sterilised between 1941 and
1943
ā¢ Not just US ā many other countries had eugenic policies,
including compulsory sterilisation, marriage restrictions, birth
control and immigration control
ā¢ Canada, Sweden, Australia, Norway, Finland, Demark and Switzerland
eugenics after WW2
Eugenics fell into disrepute
ā¢ US: Most states based eugenic sterilisation programs ended after
WWII
But, sterilisations continued for:
ā¢ Intellectual disabilities and mental illness
ā¢ Native Americans
ā¢ Black women
ā¢ Mexican women
ā¢ Prisoners
Behavioural genetics of intelligence:
estimate the degree of genetic heritability of intelligence across a population
Genetic heritability:
the degree to which variation in a phenotype (observable trait) is due to genetic variation between individuals of a given population
Heritability estimate (h2):
Average estimate of the proportion of variance for intelligence thought to be accounted for by genetic factors across a population
additive assumption
The combined effects of genes and environment are equal to the
sum of their individual effects
ā¢ G + E = 100%
ā¢ Relative strength of both ā if one is high, the other is low
ā¢ 100% - Genetic contribution = Environmental contribution
ā¢ Simplification: we know genes and environment interact (G * E)
ā¢ Considered a starting point ā used as the upper limit of genetic
heritability
Heritability estimate = 50%
ā¢ Does not mean you inherited 50% of your intelligence from your genes
ā¢ It means that 50% of the variation between people in a given population on intelligence is due to genetic factors
environment- shared/ non-shared
Shared environment (c): parents, household, teachers
ā¢ Non-shared environments (e): peer group, illnesses
assumptions about non-/shared environments
Assumed that shared environment makes the same contribution to
both identical (MZ) and fraternal (DZ) twins
ā¢ Assumed that non-shared environment is not shared
Correlation between pairs of identical twins consists of:
100% shared genes
ā¢ Identical shared environment
Correlations between pairs of fraternal twins consists of:
50% shared genes
ā¢ Identical shared environment
gene contribution-
double the difference
between the size of the identical twin
correlation and fraternal twin correlation
ā¢ (MZr ā DZr)x2
studies & their estimates of intelligence heritability
Eysenck (1979): 69%
ā¢ Herrnstein and Murray (1994): 74%
ā¢ Chipeur et al (1990): 50%
ā¢ Carroll (1993): 40%
ā¢ APA Taskforce on Intelligence (Neisser et al., 1996): 40% to 80%
haworth et al 2010: conclusions
The genetic influence of intelligence increases with age
ā¢ May account for some of the variation in heritability estimates of
intelligence
Why do genetic influences increase with age?
ā¢ In childhood, children have little say over their activities and learning
ā¢ As children grow up, they are increasingly able to select their own
experiences and may be more able to express their genetic differences
ā¢ Although it decreases with age, shared and non-shared
environment still contribute substantially to intelligence
limitations- behavioural genetic studies
Additive assumption is a simplification ā Genes and
environment can interact
ā¢ Families used in family/twin studies may not be representative
of the normal population
ā¢ Kamin and Goldberger (2002): assumption that identical and
non-identical twins have similar environments may be wrong
ā¢ Carey (2002) estimating heritability and environmental
influences from behavioural genetics
ā¢ Abstract, theoretical concepts: tell us nothing about the specific
genes or environmental influences of intelligence
ā¢ Population concepts: estimates refer to a group of people - tell us
little about individuals
davies et al 2018
Many genetic variants (SNPs) associated with intelligence
ā¢ Many genes associated with intelligence
ā¢ Each SNP/gene individually had only a very small effect
ā¢ SNP-based heritability for intelligence ranged from 12% to 25%
ā¢ Common genetic variants accounted for between 12% and 25% of the differences between people on intelligence test scores
ā¢ There was genetic overlap between intelligence and many health- related traits
ā¢ The same genetic variants associated with intelligence were also
associated with a range of physical and mental health-related traits
a complex trait
is influenced by a large number of genes
the difference in heritability estimates using behavioural techniques vs molecular
Heritability estimates are lower using molecular compared to
behavioural genetic techniques
ā¢ Behavioural genetic studies likely overestimate heritability of intelligence
ā¢ SNP-based heritability likely underestimates heritability of intelligence
is intelligence heritable?
Despite the differences in estimates, behavioural genetic and
molecular genetic studies show that intelligence is heritable
genetic influences- reduce/ increase or stay the same?
increase with age
the bell curve
Herrnstein and Murray (1994). The Bell Curve: Intelligence and Class
Structure in American Life.
ā¢ Analysis of IQ test scores in the US
ā¢ Extremely controversial book
ā¢ Huge debate
ā¢ The extent to which intelligence differences are the cause of social issues in the US
ā¢ The extent to which intelligence is genetically inherited
ā¢ Racial differences in intelligence test scores
ā¢ Whether genetic differences accounted for the racial differences in intelligence test scores
bell curve- racial differences in IQ
Wechsler Intelligence test scores:
ā¢ White Americans scored 15 IQ points higher than Black Americans
ā¢ White Americans averaged IQ of 102
ā¢ Black Americans averaged IQ of 87
ā¢ Gap between the two races is around 1 SD
ā¢ Argued that intelligence is largely genetically heritable and
therefore the racial differences in intelligence test scores are
genetic in origin
criticisms & the APA taskforce on intelligence ( Neisser et al 1996)
Large differences in the average IQ scores between Black and
white Americans
Differences in intelligence test scores may not reflect
differences in intelligence
ā¢ Tests could be culturally biased
There is no evidence to suggest these differences on
intelligence test scores between white and Black Americans are
caused by differences in genetics
ā¢ Social and economic differences
bell curve assumptions
Herrnstein and Murrayās argument:
ā¢ Intelligence is heritable
ā¢ There are ethnic differences in intelligence test scores
ā¢ Therefore, ethnic differences in intelligence are due to genetic
differences
ā¢ Taskforce agreed that intelligence is substantially heritable
ā¢ Taskforce disagreed that heritability can be used to explain
differences in IQ test scores between ethnic groups
ā¢ Heritability can only be used to explain average differences within a population group
ā¢ It cannot be used to explain average differences between groups (white Americans versus Black Americans)
issues with the bell curve
Heritability is not deterministic
ā¢ Bell curve ignored environmental influences of intelligence
ā¢ Genes and environment interact
Possible environmental explanations include:
ā¢ Bias
ā¢ Discrimination
ā¢ Poverty/deprivation
brain size and IQ
Brain size is correlated with higher intelligence
ā¢ McDaniel (2005): Meta-analysis of 37 studies (n = 1,530)
ā¢ r = 0.33
ā¢ Correlations larger in female than male participants
ā¢ Correlation larger in adults than in children
ā¢ For all ages and sex groups brain volume positively correlated with intelligence
ā¢ Race classifications do not reflect genetic similarity
Genetic variance within a race can be nearly 10x greater than the
genetic variance between races
ā¢ Between-group genetic differences do not map onto socially
recognised categories of race
ā¢ Consensus in biological and social sciences that race is a social
construct and not a genetic construct
cox et al 2019 results
Total brain volume correlated with intelligence
ā¢ r = 0.28, p < 0.001
ā¢ Examined sex differences in the relationship between total brain
volume and intelligence: No sex differences
ā¢ Grey matter:
ā¢ r = 0.28, p < 0.001
ā¢ Normal appearing white matter volume (healthy white matter):
ā¢ r = 0.25, p < 0.001
ā¢ White matter hyperintensities (damaged white matter):
ā¢ r = -0.11, p < 0.001
cox et al 2019- conclusions
Higher intelligence associated with: bigger brains, more grey
matter, more healthy white matter, less damaged white matter
ā¢ Differences in these brain indices accounted for 6.16% of the
variance in general intelligence
ā¢ Observational study: cannot disentangle directionality
ā¢ Smaller brain ā lower intelligence
ā¢ Lower intelligence ā smaller brain
ā¢ Something else cause both? (confounding)
sex differences in intelligence- johnson et al results
Two of the most population-representative samples of intelligence
ever complied
ā¢ SMS (Scottish mental survey) 1932
ā¢ No sex differences between males and females
ā¢ 0.018 standard deviation difference
ā¢ 0.28 raw points higher for female participants
ā¢ SMS1947
ā¢ Very small significant difference between males and females
ā¢ 0.109 standard deviation difference in favour of females
ā¢ 1.72 raw points higher for female participants
sex differences specific abilities
Some differences in specific abilities
Female participants tend to be better at verbal tasks
ā¢ Reading, comprehension, writing
Male participants tend to be better at some spatial tasks
ā¢ Especially mental rotation of 3D objects
These differences are small
These differences may cancel each other out when examining
general intelligence measured using a wide range of different
types of tests
sex differences- context
If/when sex differences exist, they represent the average differences between men and women, not differences between
individuals
ā¢ Knowing whether someone is male or female does not reveal
anything about their intelligence
ā¢ Many female participants will score higher than male participants
ā¢ Great variability
iodine and intelligence- results (qian et al 2005)
children receiving supplements scored 8.7 IQ points higher
than children not receiving supplements
ā¢ 3.5 years after the supplementation program was introduced,
increased to:
ā¢ 12 IQ points on non-verbal (Ravenās)
ā¢ 17.25 IQ points for verbal (Stanford-Binet)
ā¢ This study: Children who were iodine deficient
ā¢ No evidence iodine supplements improve intelligence in
children who are not deficient
education and intelligence- ritchie and tucker-drob
Meta-analysis of educational effects on intelligence
ā¢ 42 studies, >600,000 participants
ā¢ 1 additional year of education was associated with an increase in IQ of 1-5 IQ points
ā¢ Larger for ācompositeā intelligence tests (mixture of fluid and
crystallised) than for fluid tests alone.
lead exposure and intelligence
Lead is a heavy metal that was extensively used in paints,
petrol, plumbing construction
ā¢ Lead is toxic, especially to children
ā¢ Linked to damage in brain and nervous system, slowed
development
ā¢ Evidence that lead exposure in childhood has a negative impact
on intelligence.
port pirie cohort study
Largest lead smelting plant in Australia
ā¢ Amount of lead in the body and pregnancy outcomes in 749
pregnant women
ā¢ Higher levels of lead in the blood of the mothers was associated
with premature babies
ā¢ Followed-up children up throughout childhood and adolescence
ā¢ Measured levels of lead in the blood of children antenatally, at
birth, and during follow-up
ā¢ Ages 11-13: Full scale IQ scores assessed at age 11-13 with
the Wechsler Intelligence Scale for Children
port pirie results
Children with more lead exposure had lower intelligence test scores
ā¢ Lead was significantly associated with IQ at all time points, except lead levels at birth
ā¢ After adjusting for covariations (including socioeconomic status & motherās IQ) lower
IQ was associated with lead levels at:
ā¢ Age 5, Age 7 and Age 11-13
summary of influences of intelligence
Genetics influence intelligence ā Intelligence is heritable
ā¢ Although intelligence is heritable, the assumptions about the
heritability of intelligence made by eugenicists and in The Bell Curve
were flawed
ā¢ Higher intelligence is associated with brain differences (larger brains,
more grey matter, more healthy white matter)
ā¢ There is mixed evidence whether there are sex differences in
general intelligence, although there are sex differences in specific
abilities
ā¢ Environment influences intelligence
ā¢ Environmental conditions can lower intelligence (lead exposure)
ā¢ Environmental conditions can increase intelligence (iodine supplements in
those who are deficient, education)
education and intelligence in Norway
Brinch and Galloway 2012
Genetics influence intelligence ā Intelligence is heritable
ā¢ Although intelligence is heritable, the assumptions about the
heritability of intelligence made by eugenicists and in The Bell Curve were flawed
ā¢ Higher intelligence is associated with brain differences (larger brains, more grey matter, more healthy white matter)
ā¢ There is mixed evidence whether there are sex differences in
general intelligence, although there are sex differences in specific
abilities
ā¢ Environment influences intelligence
ā¢ Environmental conditions can lower intelligence (lead exposure)
ā¢ Environmental conditions can increase intelligence (iodine supplements in
those who are deficient, education)