Lecture 8 - Heritability of intelligence

Question 1

What are MZ twins?

Answer 1

• identical
• same fertilised egg
• same genetic make-up

Question 2

What are DZ twins?

Answer 2

• non identical
• 2 different eggs and 2 different sperm
• share half genetic make-up

Question 3

Twin studies and intelligence?

Answer 3

• to measure the extent of resemblance in intelligence of monozygotic twins: one compares the variation of IQ scores within a twin pair
• if identical twins are more similar to each other on intelligence test scores than non-identical twins, we can infer that genes play a role
• pattern of correlations from lots of twin study data combined suggests about 50% of intelligence various is due to genetic factors

Question 4

How do you divide the environment in twins studies?

Answer 4

• shared environment = common to both
• non shared environment = unique to each

Question 5

Claire Hanworth?

Answer 5

• took data from multiple twin studies
• twins came from all over the world and there were both MZ and DZ twins
• used the WAIS and ravens matrices test
• general intelligence score was calculated
• MZ mean = 0.78
• DZ mean = 0.51
• 54% of the differences in intelligence caused by genetic differences
• 24% of differences caused by shared environment
• 22% of differences caused by non-shared environment

Question 6

Does heritability increase with age?

Answer 6

• research has found that for intelligence heritability increases linearly from childhood through adulthood
• one theory is that environments are more similar for any given pair of 65 year olds than a pair of 4 year olds so any differences in older people must be due to their genes
• another theory is that as children grow up, they increasingly select, modify and even create their experiences, which are based on their genetic propensities

Question 7

Minnesota study of twins reared apart - Bouchard and colleagues?

Answer 7

• 139 twins reared apart
• extensive intelligence testing
• looking for genetic cascade whereby within correlations are higher for:
  -> MZ twins
  -> Lower for DZ twins
  -> Lower for distant relatives (e.g. cousins)
  -> And lowest for unrelated people (e.g. classmates)

Question 8

DNA and intelligence?

Answer 8

• there are large numbers of genetic variants in a large number of genes related to people’s intelligence scores
• using people’s DNA, one can predict better than chance the intelligence differences
• Davies et al studied peoples SNP’s and found that there were 11,600 significant SNP’s i.e. the number of single nucleotide variants related to intelligence test scores

Question 9

Definition of validity?

Answer 9

refers to the question of whether the test measures what it claims to measure

Question 10

What are the 3 types of validity?

Answer 10

1. face validity
   - an IQ test is said to have face validity if it “looks like” it is going to measure what it is supposed to measure
2. concurent validity
   - intelligence test shows concurrent validity if it shows a (positive) relationship with other measures of intelligence
3. predictive validity
   - intelligence tests have been used to predict ‘real-world‘ measures of intelligence or achievement (e.g. school achievement, job performance, etc.)

Question 11

What are the 2 aspects of reliability?

Answer 11

1. Internal reliability
   -> Any measure of intelligence with good internal reliability will have a number of items that correlate positively with one other, this suggests they are measures of the same construct
2. Test-retest reliability
   -> A good intelligence test will show a good level of reliability over time
   -> Your general intelligence is thought to be relatively stable over time
   -> You would expect IQ scores to be very similar across test sessions

Question 12

The Flynn effect?

Answer 12

• scores on intelligence tests tend to fluctuate
• scores of intelligence tests change continuously from year to year
• Flynn discovered a year on year rise of intelligence test scores whenever a new IQ test was compared to an older one
• He found that in 24 years the IQ gain from WISC to WISC-R was 8 points
• looked at 3 studies

Question 13

Results of Flynn’s 1987 meta analysis?

Answer 13

• highest rise in IQ = non verbal tests (fluid intelligence)
• lowest gains = verbal tests (crystallised intelligence)

Question 14

Explanations of the Flynn effect?

Answer 14

• generations getting more intelligent
• people becoming more familiar with IQ tests
  -> doesn’t explain the difference between verbal and non verbal IQ tests
• parents becoming more interested in their child’s development