Human Intelligence

Human Intelligence

1. Evolutionary factors

   • Change in diet promoted brain development
     • Meat is thought to have provided an evolutionary advantage - more nutrient rich diet
       • Shipman & Walker - meat became part of the diet ~ 3 million years ago
       • Molars shrank before brain growth - meat made the brain grow
         • Leonard - homo erectus molar SA 380mm, homo sapien 320mm
       • Provided essential AAs & ions
       • Made favourable by natural selection
       • Excess energy used for brain growth & development
       • Properties of meat meant humans had to eat less
     • Brain became more convoluted, more folds, more intelligence
   • Humans harnessed fire ~ 500,000 years ago
     • Cooking made food easier to chew, required less effort to digest food
       • Able to extract more nutrients
       • Molar size decreased
       • Kaitovich - after cooking, humans started making new & more sophisticated tools
         • This lead to new & better tools being created, increased ingenuity, technology & culture
         • Cooked food didn't require energy to bring up to body temperature
   • Limiting factor, brain size limited by size of the birth canal
   • Khaitovich et al - 200,000 years ago, gene for metabolic function rapidly evolved. Increasing brain energy expenditure
   • Foraging demands
     • Frugivores had to increase cognitive functioning to maintain food supply only available at certain times+places
     • Tool use indicator of intelligence, used to extract food
       • Mercader et al - evidence of tool use in chimps
       • More successful hunter-gatherer groups use more sophisticated tools than less successful groups
     • Beals - eastern asians have larger cranial volume, more demanding environment to survive. Larger brains needed - Out of Africa hypothesis
       • Peters - male brains heavier than females, no difference in IQ
         • Gender bias
         • Brain organisation more important, female brain more organised
   • Social complexity
     • Intelligence & brain size greater in species with complex social structures
     • In response to demands of group living inc. ToM & Machiavellian intelligence
     • Dunbar - strong correlation between social group size & neocortex size. Not with environement
     • Schillaci - largest brain sizes in monogamous mating systems. Monogamous relations require complex social skills
     • Homo sapiens group size ~ 150, intelligence important for survival within a group
     • Mitani & Watts - male chimps more likely shared meat with other males, than females. Formed alliances, more successful future hunting. Machiavellian intelligence
       • Meat also used for sex: Hill & kaplan - Ache people (Paraguay) meat shared outside group. Skilled hunters get disproportionate sexual favours
     • Great diversity in primate social systems. Orang-utans have ToM - live in small groups
       • Ants, no ToM, live in large social groups
     • Primates brains should be 3x larger if group living is a main factor
   • Issues with a large brain
     • Require lots of energy & compete with body's other organs for resources
     • Takes longer to mature, greater dependence on parents for longer, limits minimum reproductive age
       • Selective pressures against developing a larger brain
     • Large brains not exclusive to humans, primates have similar sized brains (in ratio) yet lack langage

2. Genetic factors in test performance

   • Twin & adoption studies
     • Bouchard & McGue - IQ concordance 60% DZ raised together 72% MZ raised apart
     • Malouff et al - meta analysis, half variation in IQ scores due to genetics
     • The Texas Adoption Project (Horn et al) - adopted children studies 10 years apart 1st IQ closer to adopted parents, 2nd IQ closer to genetic parents. Family impact decreases with age, genetics increase
     • Kamin & Goldberger - twin studies overestimate inheritability, MZ twins more similar due to similar environment
     • Voracek & Haubner - twin & single births IQ tested, twins 4 points lower
     • Tang et al - mice genetically modified to boost memory and learning, same could be done in humans
     • Turkheimer et al - well of children have ~ 60% due to genetics, poorer families have little/no genetic influence
     • Genetic engineering, can be used to alter human DNA if a gene for intelligence is found to raise intelligence
     • Genes may cause us to seek certain experiences not others
     • Children often adopted into similar families - explains high correlations
   • Genetics
     • Hill et al - x2 super bright group children had IGF2R variant
       • Less than half super bright group had this variant, some average group had it environmental & genetic factors make up final intelligence
         • Discovery of 'intelligence genes could lead to discrimination against those who don't have it
     • Caspi et al - children with FADS2 variant had IQ scores 7 points higher if they were breastfed. Children with another variant showed no improvement
       • Genes can alter environmental influences
       • Diathesis stress

3. Environmental factors in test performance

   • Family enrionment
     • Socioeconomic status: Class I (professional) - Class V (unskilled)
       • Mackintosh - after financial harship & area of residence are accounted, class I children score average 10 IQ more than class V
       • Higher SES - able to provide children with better resources. Essential to cognitive development
       • Can be used to give adopted low SES children a good chance
         • Schiff et al - low SES children adopted in high SES, improved IQ scores
       • Low SES children showed lower PFC response to novel stimuli, possible consequence of stressful, poor environment
     • Birth order & family size
       • Belmont & Marolla - family size in 386 19 year old Dutch men. Larger families had lower IQ than smaller families. First born child had typically higher IQ
       • Bjerkedal et al - birth order effect found in Norway. Forst born highest IQ, when first child died, 2nd child IQ close to 1st average
       • Blake - 100,000 people in small/medium families, birth order had no effect on progression in education. Birth effects don't persist
   • Culture
     • Group socialization theory - experiences outside the home more important for developing intelligence than those in the home.
       • As someone gets older they become less dependent on family life, more influenced by peers
         • Pinker - gens & peers have some influence on behaviours such as smoking, but not on intelligence
     • Herrnstein & Murray - US immigrants had lower IQ than nationals, immigration caused a downwards pressure on intelligence. Increase in social problems associated with low IQ
       • Greenfields - IQ testing someone from another culture promotes cultural bias in test scores - imposed etic
     • Ceci - meta analysis to see if school improved IQ. Higher IQ for more regular attendance & IQ decreases over summer holidays. Average rise 2.7 per school year
       • Herrnstein & Murray - intelligence mostly inherited genetically, no one can manipulate IQ significantly through environment & schooling
     • Head start - 1960s project provided compensation education to underprivileged children. Some initial IQ gains, long term effectiveness only in some individuals
       • Perry preschool project - high quality preschooling for African-American children in poverty with high risk of school failure. Starting school 67% individuals in the programme had IQ > 90, only 28% not in programme had IQ > 90