Intelligence 2

• Heritability

  The proportion of variation in a trait that can be attributed to genes, and not the proportion of a trait caused by genes
• It has been estimated that between 40-80% of current population variance in intelligence scores is due to genetic factors
• Virtually no psychologists dispute the claim that intelligence is strongly influenced by genetics
• The "Wilson effect" - Heritability may be higher (.7-.8) in adults compared to children (.45)
• Caveats around heritability

  • If environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability
  • A common error is to assume that a heritability figure is necessarily unchangeable
  • Heritability includes both direct effect of genotype on intelligence and indirect effects where genotype changes the environment, which could lead to overestimation of direct genetic effects and underestimating the environment
• Flynn Effect


  • IQ norms were changing over time such that when people did older intelligence tests, they would have received a higher IQ
  • Intelligence was rising around 3IQ points per decade from 1932 to 1978
  • Biggest rise in non-verbal (Gf) tests
  • Verbal IQ rise (Gc) 9 points per generation
  • Non-verbal IQ rise (Gf) 15 points per generation
  • Flynn effect may be slowing down or reversing
• Environmental effects on IQ

  • Nutrition
  • Prenatal factors
  • Smoking
  • Alcohol
  • Other drugs e.g., cocaine, opioids
  • Family environments
• Bidirectional relationship between intelligence and how children might be treated


  Family environments might enhance or diminish differences between siblings
• Correlation does not equal causation. E.g., parents with higher intelligence pass both (a) genes and (b) environment down to children
• Breastfeeding and intelligence

  Good source of nutrients and bioactive compounds that may support brain development and function
• Mixed effects for brain training. After adjusting for publication bias, only the effect size for processing speed remained significant. Transfer to attention, objective everyday functioning, fluid-intelligence, and visuospatial domains was not significant.
• Bidirectional relationship with education


  Controversy over effects of early enrichment programs - Gains from Head Start program disappeared after a few years
• There is mixed evidence for sex differences in intelligence
• Sex differences in intelligence

  • Slight male advantage on g beginning in adulthood
  • Medium sized advantage for males on spatial intelligence
  • Advantage in emotional intelligence in women
  • Mixed evidence for female advantage in verbal intelligence
  • Greater variability in males - more males at both very high and very low ends of the distributions
• Possible reasons for sex differences

  • Biological - Hormones, evolutionary arguments
  • Environmental - Stereotypes and stereotype threat
  • Socialisation/learning experiences
• There is substantial evidence for 'race' differences in intelligence test performance but also substantial debate about the causes
• Possible causes of race differences in intelligence


  • Test-bias
  • Genetic component
  • Environmental component
  • What is 'race'? Social constructed groups - huge amount of genetic variation within these groups
• There is no real scientific consensus on the existence, magnitude and causation (e.g., genes or environment) of race and sex differences in general intelligence
• Bell Curve

  • Argued against social programs for low intelligence people and groups as inefficient use of resources
  • Class is and will become even more so determined in part by genetic factors such as intelligence
• An acknowledgement of genetic factors could also support progressive policies. Also, hard to determine whether poverty causes low intelligence or low intelligence causes poverty.
• If intelligence is highly predictive (and probably causal) of positive life outcomes, is there a moral imperative to encourage breeding of highly intelligent individuals and discourage breeding of people of low intelligence?
• Modern intelligence tests are well-validated, non-biased measures of intelligence
• g


  A general factor of intelligence
• Other researchers have introduced models such as Gf/Gc theory, and the CHC three stratum theory