background

• intelligence: generally understood as the ability to acquire and use skills - there isn't one objective definition as it depends on the skills being tested
• Boring (1923) definition of intelligence: intelligence comes to represent whatever intelligence tests are testing at that time period
• modern IQ tests focus on: logical reasoning
• single factor Spearman's 'g'

  intelligence can be understood as a single general factor ('g'), responsible for success across a multitude of different mental tasks. likely to account for 50% of variance across mental tasks (ie 50% of success in tasks could be attributed to g and 50% to other factors eg determination for success, support from teacher)
• how did Spearman come to single factor theory

  he studied relationships between cognitive factors in school children, finding consistent correlation in performance across a number of unrelated academic subjects
• carroll's three-stratum model (1993)

  suggests a 3 level hierarchy of intelligence: stratum I, stratum II and stratum III
• stratum I

  69 narrow cognitive abilities
• stratum II

  gathers narrow abilities into 8 broad ability factors (including fluid intelligence and crystallised intelligence)
• stratum III

  spearman's g factor
• halpern (1997) - sex differences 


  found evidence of differences in cognitive abilities between sexes eg women achieve higher scores on verbal fluency tasks and foreign language tasks while men achieve better scores in mental rotation tasks and scientific reasoning
• haier et al (2005) - sex differences neural anatomy


  using MRI scans, they found that men have more grey matter in frontal and parietal lobes (typically associated with motor skills and higher level reasoning) while women have more grey matter in different areas of frontal lobe and Broca's areas (important for tasks like speech and writing)
• van goozen et al (1995) - sex differences sensitivity to hormones


  studied female-to-male transsexuals who were taking testosterone and found significant increase in visual spatial ability and lowered verbal fluency - opposite effect found in male-to-female patients
• neural explanations however...

  men and women were found to achieve similar IQ results despite differences in brain regions as tests often involve mixture of cognitive tasks. neurological evidence currently suggests that there is no single underlying neuroanatomical structure for general intelligence
• heredity


  children inherit characteristics/traits from their parents , as genes are passed onto offspring from parents
• scarr & weinberg (1978) - genetic factors


  compared intellectual abilities of parents to their adopted vs biological offspring - found stronger correlations between biological relatives than adopted ones, suggesting hereditary basis of intelligence (nature over nurture)
• plomin & defries (1998)


  compared monozygotic (identical) and dizygotic (fraternal) twins on their spatial and verbal skills, finding similarity between monozygotic twins was stronger - even with shared environment
• scarr (1997) - genetic factors


  even when reared apart, monozygotic twins have higher concordance rates than dizygotic twins who were reared together
• desrivieres et al (2014)


  found that NPTN (neuroplastin) gene links to some variation finding that in average those carrying a particular variant of the gene have a thinner layer of grey matter in the left cerebral hemisphere and performed less well on IQ test
• problems with twin studies

  *fail to take into account impact of assortative mating (ie assume that parents phenotypes are uncorrelated)

  *fail to take into account interaction between genes and environment

  *ignore genotype-environment correlation