van Leeuwen et al (2008) A twin-family study of general IQ

• assortative mating

  individuals with similar genes or observable characteristics mate with one another more frequently than those who do not (more often than expected if mating were purely random)
• phenotypic assortment

  assortative mating occurs because individuals choose one another because they have similar intelligence levels
• social homogamy


  people with similar intelligence levels are clustered together in the same environment they are more likely to end up having children together - individuals are attracted to individuals from a similar social/cultural background
• genetic transmission

  transfer of genetic information from genes to another generation
• cultural transmission

  the way a group of people within a society or culture tend to learn and pass on information
• gene-environment interaction

  theory that certain environments 'activate' a particular gene
• gene-environment correlation

  parents transmit their genes and their environment to their children. for example parents maybe predisposed to good health and fitness, but also share this lifestyle to their children. therefore it makes it difficult to establish the contribution of nature or nurture
• heritability

  the extent to a parents genes are responsible for a phenotype
• the aim was

  to measure the relative influence of assortative mating, cultural transmission, and GE interaction & GE correlation. they also wanted to investigate why spouses have similar intelligence scores to one another (phenotypic assortment or social homogamy)
• research method

  research article using extended twin design (twins, their siblings and parents) - partly correlational study
• sample

  *twins drawn from the Netherlands Twin Registry (NTR) at VU University of Amsterdam

  *214 twin families (twins about to turn 9 years old) with an extra sibling between 9 and 14 years were selected from two birth cohorts (1995-1996) and invited to participate by letter. families with children with a major medical history were excluded (eg psychiatric problems, major medical issues, special educational needs or physical/sensory disabilities)

  *112 families agreed to participate, 103 of which had full siblings who wanted to participate

  *mean age of twins - 9.1 years

  *mean age of siblings - 11.9 years (range 9.9 - 14.9 years)

  *mean age of mothers - 41.9 years

  *mean age of fathers - 43.7 years

  (both parents mid 40s mean age)
• procedure

  parents signed informed consent forms for their children and themselves. children also signed their own consent forms. parents were compensated for their travel expenses and children received a present. cheek swabs, for DNA, or a questionnaire were collected at home by parents and children prior to the intelligence testing to determine whether the children were identical twins or non-identical twins (confirmation of zygosity). for cognitive/intelligence testing, families arrived in the morning. children were tested in separate rooms with Raven's Standard Progressive Matrices (SPM). they were given verbal instructions and completed the test at their own pace. test consisted of 60 problems divided into 5 sets of 12 questions that become increasingly difficult as the test progresses. the test covers a range of cognitive abilities, from identifying missing puzzle pieces to completing analogies. adults completed a more advanced version known as Raven's Advanced Progressive Matrices (APM). they were given written instructions and also worked at their won pace. although the number and arrangements of questions and sets were slightly different, both types of test produced comparable results (reported retest reliability for adults is 0.91). whole protocol took approximately 5 hours, including two short breaks and one longer lunch break. in their analysis of data, researchers used two different theoretical models to determine whether spousal resemblance is better explained by phenotypic assortment or social homogamy.
• results

  *RPMs were used to assess general intelligence (IQ) and a persons IQ was estimated using a Rasch model (accounts for the level of difficulty in the questions, so scores weren't solely calculated on amount of correct answers - more difficult questions had higher weightings)

  *no significant differences in IQ scores between males and females (sex differences) across all groups, and within individual groups (parents, siblings and twins)

  *the variance in the siblings was significantly larger than in the twins - siblings had wide ranging scores on the test, whereas twins had very similar scores. supports a genetic explanation, because the more genetic makeup the share, the more similar their IQ is

  *most of the findings supported genetic explanations for IQ level, so cultural/environmental factors were not significant on their effects on IQ level

  *Spousal correlation for Rasch IQ estimates was moderately high (0.33) - i.e if one had high score so did the other. provides evidence for assortative mating (phenotypic assortment), as it suggests that individuals seek and select partners with similar intelligence rather than ending up selecting a mate with similar IQ because they're in the same social circle

  *correlations between Raven test scores were higher for identical twins, than sibling, and non-identical twins - suggests intelligence is inherited

  *environmental factors were found to be more important in children with a genetic disposition for low-IQ groups than those with a disposition towards high-IQ groups (i.e children born with low IQ can be effected more by environmental factors compared to children born with high IQ) - provides evidence for GE interaction where genetic and environmental factors affect one another to determine intelligence levels. people with certain genotype (lower IQ) are more sensitive to environment
• Raven's progressive matrices

  one of the most popular non-verbal tests of intelligence in current use. aims to test 'eductive reasoning' - ability to infer/work out an answer based only on the info one has been given. provides an incomplete, novel pictorial stimuli which participants have to make meaningful - for each item ppts have to select the missing aspect in a series of stimuli to complete a pattern. it tests fluid intelligence. there's 3 versions (CPM, SPM & APM) and in each the test gets progressively more challenging. it is also 'culture fair' (doesn't have cultural bias)
• fluid intelligence


  the ability to reason and solve problems using new information without relying on previously acquired knowledge and skills - ability to deal with novelty and adapt one's thinking 'fluidly' to a new, unfamiliar problem
• crystallized intelligence


  ability to use learned knowledge and experience
• conclusions

  1. the main influence on IQ level is genetic factors, however genes do interact with environmental factors to influence intelligence in significant ways

  2. cultural transmission does not have a significant influence on IQ level

  3. phenotypic assortment better explains spousal resemblance that social homogamy