Human Intelligence

Evolutionary factors
1. Change in diet promoted brain development
  1. Meat is thought to have provided an evolutionary advantage - more nutrient rich diet
    1. Shipman & Walker - meat became part of the diet ~ 3 million years ago
    2. Molars shrank before brain growth - meat made the brain grow
      1. Leonard - homo erectus molar SA 380mm, homo sapien 320mm
    3. Provided essential AAs & ions
    4. Made favourable by natural selection
    5. Excess energy used for brain growth & development
    6. Properties of meat meant humans had to eat less
  2. Brain became more convoluted, more folds, more intelligence
2. Humans harnessed fire ~ 500,000 years ago
  1. Cooking made food easier to chew, required less effort to digest food
    1. Able to extract more nutrients
    2. Molar size decreased
    3. Kaitovich - after cooking, humans started making new & more sophisticated tools
      1. This lead to new & better tools being created, increased ingenuity, technology & culture
      2. Cooked food didn't require energy to bring up to body temperature
3. Limiting factor, brain size limited by size of the birth canal
4. Khaitovich et al - 200,000 years ago, gene for metabolic function rapidly evolved. Increasing brain energy expenditure
5. Foraging demands
  1. Frugivores had to increase cognitive functioning to maintain food supply only available at certain times+places
  2. Tool use indicator of intelligence, used to extract food
    1. Mercader et al - evidence of tool use in chimps
    2. More successful hunter-gatherer groups use more sophisticated tools than less successful groups
  3. Beals - eastern asians have larger cranial volume, more demanding environment to survive. Larger brains needed - Out of Africa hypothesis
    1. Peters - male brains heavier than females, no difference in IQ
      1. Gender bias
      2. Brain organisation more important, female brain more organised
6. Social complexity
  1. Intelligence & brain size greater in species with complex social structures
  2. In response to demands of group living inc. ToM & Machiavellian intelligence
  3. Dunbar - strong correlation between social group size & neocortex size. Not with environement
  4. Schillaci - largest brain sizes in monogamous mating systems. Monogamous relations require complex social skills
  5. Homo sapiens group size ~ 150, intelligence important for survival within a group
  6. Mitani & Watts - male chimps more likely shared meat with other males, than females. Formed alliances, more successful future hunting. Machiavellian intelligence
    1. Meat also used for sex: Hill & kaplan - Ache people (Paraguay) meat shared outside group. Skilled hunters get disproportionate sexual favours
  7. Great diversity in primate social systems. Orang-utans have ToM - live in small groups
    1. Ants, no ToM, live in large social groups
  8. Primates brains should be 3x larger if group living is a main factor
7. Issues with a large brain
  1. Require lots of energy & compete with body's other organs for resources
  2. Takes longer to mature, greater dependence on parents for longer, limits minimum reproductive age
    1. Selective pressures against developing a larger brain
  3. Large brains not exclusive to humans, primates have similar sized brains (in ratio) yet lack langage
Genetic factors in test performance
1. Twin & adoption studies
  1. Bouchard & McGue - IQ concordance 60% DZ raised together 72% MZ raised apart
  2. Malouff et al - meta analysis, half variation in IQ scores due to genetics
  3. 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
  4. Kamin & Goldberger - twin studies overestimate inheritability, MZ twins more similar due to similar environment
  5. Voracek & Haubner - twin & single births IQ tested, twins 4 points lower
  6. Tang et al - mice genetically modified to boost memory and learning, same could be done in humans
  7. Turkheimer et al - well of children have ~ 60% due to genetics, poorer families have little/no genetic influence
  8. Genetic engineering, can be used to alter human DNA if a gene for intelligence is found to raise intelligence
  9. Genes may cause us to seek certain experiences not others
  10. Children often adopted into similar families - explains high correlations
2. Genetics
  1. Hill et al - x2 super bright group children had IGF2R variant
    1. Less than half super bright group had this variant, some average group had it environmental & genetic factors make up final intelligence
      1. Discovery of 'intelligence genes could lead to discrimination against those who don't have it
  2. Caspi et al - children with FADS2 variant had IQ scores 7 points higher if they were breastfed. Children with another variant showed no improvement
    1. Genes can alter environmental influences
    2. Diathesis stress
Environmental factors in test performance
1. Family enrionment
  1. Socioeconomic status: Class I (professional) - Class V (unskilled)
    1. Mackintosh - after financial harship & area of residence are accounted, class I children score average 10 IQ more than class V
    2. Higher SES - able to provide children with better resources. Essential to cognitive development
    3. Can be used to give adopted low SES children a good chance
      1. Schiff et al - low SES children adopted in high SES, improved IQ scores
    4. Low SES children showed lower PFC response to novel stimuli, possible consequence of stressful, poor environment
  2. Birth order & family size
    1. 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
    2. Bjerkedal et al - birth order effect found in Norway. Forst born highest IQ, when first child died, 2nd child IQ close to 1st average
    3. Blake - 100,000 people in small/medium families, birth order had no effect on progression in education. Birth effects don't persist
2. Culture
  1. Group socialization theory - experiences outside the home more important for developing intelligence than those in the home.
    1. As someone gets older they become less dependent on family life, more influenced by peers
      1. Pinker - gens & peers have some influence on behaviours such as smoking, but not on intelligence
  2. Herrnstein & Murray - US immigrants had lower IQ than nationals, immigration caused a downwards pressure on intelligence. Increase in social problems associated with low IQ
    1. Greenfields - IQ testing someone from another culture promotes cultural bias in test scores - imposed etic
  3. 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
    1. Herrnstein & Murray - intelligence mostly inherited genetically, no one can manipulate IQ significantly through environment & schooling
  4. Head start - 1960s project provided compensation education to underprivileged children. Some initial IQ gains, long term effectiveness only in some individuals
    1. 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

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Human Intelligence

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