MA Lecture 5 - Nature or Nurture

Is Intelligence Heritable?

Heritable — whether a trait or characteristic can be passed from parents to their offspring through genes 

• in the context of intelligence — proportion of differences in intelligence among people in a popu can be explained by genetic differences 

• Galton — Hereditary Genius 

  • analysis of genealogical trees of 145 eminent persons, judges, politicians etc 

Francis Galton Findings

Prominent people have prominent relatives

• closer the kinship — more likely relative also prominent 

• conclusion — genius & feeble mindedness runs in families

Study of Hereditary Intelligence — Twins

Galton — History of Twins

• used twins to try & distinguish between the effects of nature (heredity) and nurture (environment) 

• wrote to twins & relatives of twins asking about similarities and differences 

• 80 cases of twins being v similar to one another and 20 dissimilar 

• concluded that hereditary tendencies were more powerful than environ factors in shaping personality & intellect 

Since Galton — Adoption & Twin Studies 

• study genetically related individuals raised apart (e.g. biological parents & adopted children out; separated twins) 

• similarity can be more likely attributed to genetic factors

• study genetically unrelated individuals raised together (e.g. adoptive parents & children) 

• similarity can be attributed to shared environment 

• compare w normal family situation — parents and children who share genes and environment 

DZ vs MZ Twins

Monozygotic Twins — share 100% of genes

• raised together — similarity = genes + shared environment

• raised apart — similarity = genetic influence isolated

Dizygotic Twins — share 50% of genes

• raised together — similarity = genes + shared environment

most twin studies compare MZ and DZ twins raised together

• if MZ twins are more similar to each other in intelligence than DZ twins — suggests genes play a major role

Adoption Studies

Biological relatives raised apart

• similarity = genetic influence

Unrelated individuals raised together

• similarity = shared environmental influence

Concordance Rates 

percentage of pairs (usually twins or family members in which both individuals exhibit the same trait 

Heritability (H) 

the proportion % of phenotype variation in a tested sample of people that can be accounted for by genetic variation 

• heritability estimates apply to a group at a given time not individuals 

• if the heritability of intelligence is 50% in a group then half of the observed differences in intelligence between people in that group are due to genetic differences 

• not that 50% of an individuals intelligence is genetic 

• helps us understand what causes differences between people not what causes a trait in an individual 

Genotype — Underlying genetic factors (specific DNA) 

Phenotype — expression of underlying genetic factors 

Twin Studies — Meta Analysis

Polderman

• heritability is 49%

Limitations of Twins and Adoption Studies

equal environments assumption (living together)

• assumes MZ and DZ twins experience equally similar environments

• MZ twins may be treated more similarly due to identical appearance — inflates genetic estimates

selective placement in adoption

• adopted children may be placed in families similar to their biological background (SES, edu level)

• confounds genetic vs environmental effects

non-shared environment

• even within same household — individuals experience unique environments

Genome-Wide Association Studies

• heritability analyses estimate genetic influence on a trait not measuring specific genes

• GWAS of intelligence

  • genetic similarity is measured rather than inferred

  • early tests — 20 & 30% heritability

  • more recent tests — 54% DNA based heritability of intelligence

  • actual mechanisms of underlying intelligence still unknown

Environmental Factors and Intelligence 

many environ factors linked to increasing intelligence 

• edu

• SES — adoption studies — lower to higher SES → higher IQs

• Nutrition 

• SES positively correlates w intelligence scores

• SES-intelligence association partly through access to better nutrition and health knowledge 

• breastfeeding linked to IQ gains — up to 3.5 IQ points higher 

The Flynn Effect 

observed rise over time in standardised intelligence test scores 

• Flynn found a 13.8 point increase in IQ scores between 1932 and 1978

  • 3 points per decade

  • supported by later data

  • avg annual increase — 0.31 IQ points

• from 1918-1995 US Ss gained almost 25 IQ points

Flynn Effect — Meta Analysis

• Highest gains for Gf > Spatial > Gc 

• Implications 

  • an average person of 1910 compared to today would have a mean IQ of 70 (intellectual disability)

  • need to keep re-calculating average IQ

Causes of Flynn Effect

• environ possibilities

  • improved schooling

  • better nutrition

  • tech

  • lead reduction

  • modernisation

• Flynn — IQ tests measure abstract problem solving ability

  • practical vs abstract importance

  • modernised people have become more familiar w abstract concepts