INTELLIGENCE, THINKING AND PERSONALITY

1
INTELLIGENCE, THINKING AND PERSONALITY

• Genetic and Environmental Factors in Intelligence

2
THE HEREDITARIAN LEGACY

• In the section on testing, we saw that early US
  workers were hereditarians.
• They saw intelligence as a (relatively) fixed
  characteristic of a person that was largely
  inherited.
• The same is true of Spearman and his heirs in the
  UK - Burt, Vernon, Eysenck.
• Politically, the hereditarian position is often
  associated with a desire to maintain the social
  status quo.

3
THE HEREDITARIAN LEGACY- JENSEN

• More recently Jensen used the fact that remedial
  educational programmes such as Headstart were
  apparently failing to argue that blacks' IQ
  couldn't be improved much by environmental
  factors.
• Jensen also argued that the tests used were not
  biased (Bias in Mental Testing, 1980).
• If they are, there might be nothing to explain
• His argument are directed to a technical sense of
  bias
• different intercepts for blacks and whites on a
  regression line (e.g. of achievement on IQ)
• He did not address the issue of mean differences
  of groups on the same regression line.

4
THE HEREDITARIAN LEGACY - THE BELL CURVE

• Arguments similar to Jensens are made in the
  influential 1994 book The Bell Curve by
  Herrnstein and Murray
• Gould argues that there is little that is new in
  this book and that the same problems affect its
  arguments as affect Jensens

5
NATURE versus NURTURE

• From the simple observation that children are
  relatively similar in intelligence (or at least
  IQ) to their parents it is difficult to
  disentangle the contributions of inheritance and
  environment
• Children share genes with their parents, but they
  also, almost always, share environment
• So, either or both could contribute to similar IQs

6
PHENOTYPE and GENOTYPE

• Physical and psychological characteristics are
  part of an animals or persons phenotype
• But an organism can only develop in an
  environment, so even if a characteristic is
  inherited, the genotype only produces that
  characteristic by using environmental
  resources/conditions
• Nevertheless, within a certain set of varying
  environments, genetic factors can make a stronger
  or a weaker contribution

7
HERITABILITY

• The notion of heritability is a technical one.
• It is not directly applicable to individuals but
  is about explaining variance within a group
  (variance not mean level of performance).
• Technically it is the proportion of the
  phenotypic variation in a trait that is
  attributable to genetic variation

8
HERITABILITY - cont

• There is an issue of what constitutes the
  group.
• But in any case, heritability is not a measure of
  between-group differences.

9
COMPARING RELATIVES

• An obvious comparison in investigating
  heritability of human characteristics is of
  people who are more versus people who are less
  closely related.
• However, as previously noted, closely related
  people (especially, children raised in the same
  family) share an environment.

10
NATURE versus NURTURETWO CRITICAL COMPARISONS

• Monozygotic (MZ) versus dizygotic (DZ) twins
• Reared together (shared environment) versus
  reared apart (less similar environments)
• Adopted versus biological children
• Within the same family, they share environment

11
MONOZYGOTIC (MZ) VERSUS DIZYGOTIC (DZ) TWINS

• MZ twins result from splitting of a single
  fertilised egg (zygote) and share 100 of their
  genes
• DZ twins result from two separate fertilised eggs
  and share on average 50 of genes, like ordinary
  siblings

12
MONOZYGOTIC (MZ) VERSUS DIZYGOTIC (DZ) TWINS

• If brought up in the same family MZ and DZ twins
  both share very similar environments
• But are they the same? And are the environments
  of MZ twins more similar than those of DZ twins,
  even in the same family?
• MZ twins may be treated more similarly than DZ
  twins or siblings (for example because they are
  perceived to be more similar)
• This would be an interaction between genes and
  environment.

13
EMPIRICAL STUDIES OF TWINS

• Main Studies
• Newman, Freeman and Holzinger, 1937
• Burt, 1955, 1958, 1966
• Shields 1962
• Juel-Nielson, 1965
• Minnesota twin study (Bouchard et al. ongoing)
• Apart from last, suffer from small samples
  (particularly of MZ reared apart)

14
MZ TWINS REARED APART- THE CRUCIAL CASE

• MZs reared apart supposedly have same genes but
  different environment, therefore similarities
  should have a genetic explanation
• They are usually compared with same sex DZs
  reared together.
• However, it can be difficult to find cases in
  which the environments are not similar
• Minnesota twin study (Bouchard et al) - reports
  high correlations for IQ scores of MZ twins
  reared apart.

15
TYPICAL RESULTS

• Correlations of IQs of
• MZ twins reared together 0.85
• MZ twins reared apart 0.76
• DZ twins reared together 0.60
• Siblings, or parents and children 0.5
• 1 vs 3 suggests a genetic component
• 1 vs 2 suggests an environmental component

16
PROBLEMS WITH SPECIFIC CASES OF MZs REARED APART

• Not always separated for long (e.g Shields
  specified 5 years, and some were at school
  together anyway)
• Often placed in matched families
• Experimenter bias - Newman and Shields didn't use
  blind techniques

17
PROBLEMS - cont

• Sample bias - earlier studies couldn't use
  genetic tests for MZ so may have wrongly
  classified MZs (as DZs) if they appeared
  different or had different personalities
• Tests used weren't the same in the different
  studies and weren't always satisfactory (e.g
  Newman used the original Stanford-Binet on adults
  even though the test is only standardised for
  children)
• Burt (almost certainly) invented some of his data

18
TWIN STUDIES AND THE ENVIRONMENTAL VIEW

• Scarr and Carter-Saltzman,1979, showed that
• For a surprising number of twins, beliefs about
  whether they were MZ or DZ were incorrect.
• When the two did not coincide, genetic tests of
  MZ were a better predictor of similarity in IQ
  than twins' beliefs about whether they were
  identical
• This finding shows that the argument about more
  similar environments for MZ (MZ twins treated
  more similarly because they are thought to be
  more alike, and this more similar environment
  affecting IQ) can be only partly valid

19
TWIN STUDIES AND THE ENVIRONMENTAL VIEW

• A further problems is that the environmental
  position suggests that DZ twins should be more
  like MZ twins than siblings are
• They are (see correlations) but the gap between
  MZ and DZ twins is greater than the gap between
  DZ twins and siblings

20
ADOPTED VERSUS BIOLOGICAL CHILDREN

• Biological children share 50 of genes with each
  parent
• Adopted children share fewer genes with adoptive
  parents
• Biological and adoptive children within the same
  family share environments
• But are those environments the same or do parents
  treat biological and adoptive kids differently?

21
FOSTERING AND ADOPTION STUDIES

• Early Studies
• Burks, 1928
• Leahy, 1935
• Skodak and Skeels 1949
• Typically report a correlation of about .15
  between adoptive parents and children compared
  with about .5 for biological parents
• Horn et al 1975 report .15 vs .32

22
FOSTERING AND ADOPTION STUDIES - cont

• But adoptive parents are a more homogeneous group
  - so less scope for correlation
• Also, adoptive homes tend to be "better" homes
  and absolute values of kids IQs of kids move up
  considerably (Schiff et al., 1978
• French working-class kids adopted into
  middle-class families
• However, absolute values are not important in
  determining correlation
• If better environment has a uniform effect it
  doesnt affect variability in IQ, which is what
  is nature versus nurture debate is all about
• malnutrition in early childhood is similarly
  important (Brown and Pollitt, 1996 -Guatemala
  project)

23
BETWEEN- AND WITHIN-GROUP DIFFERENCES

• Heritability is concerned with within-group
  differences (in IQ or whatever)
• As has often been pointed out, between group
  differences may have a totally different
  explanation from within group differences
• One could be genetic, the other environmental.
• The oft-cited (biological) example comes from
  Lewontin (1976).

24
LEWONTINS EXAMPLE

• Two handfuls of mixed seed are planted one in a
  good bed of soil and one in a poor bed.
• Within-group (bed) differences (e.g. in height of
  plants) are genetic, group mean difference is
  environmental (plants grow taller in better
  soil).
• Can this example be generalised to intelligence?

25
EXTENDING THE EXAMPLE?

• In more realistic situations, genetic and
  environmental influences cannot be so easily
  separated.
• And in any case, as biologists such as Gould are
  keen to point out, phenotypes are the result of
  an interaction between genotype and environment.

26
EXTENDING THE EXAMPLE?

• A further problem (which partly invalidates
  hereditarian arguments), particularly in the US,
  is that it is very hard to eliminate the
  possibility of environmental contributions to
  between-group differences
• For example, it might be thought that equating
  blacks and whites for socio-economic status would
  control for the generally poorer environment of
  blacks
• However, this ignores the fact that even high SES
  black may experience discrimination in a way that
  whites do not

27
SOME ILLUMINATING DATA FROM OUTSIDE THE USA

• Loehlin et al., 1975, compared illegitimate
  children of US service personnel in Germany,
  black vs white (white mothers, roughly comparable
  socio-economically fathers not present)
• they found little difference in IQ between the
  two groups

28
DICKENS AND FLYNN 2001HOW TO RECONCILE LARGE
ENVIRONMENTAL EFFECTS WITH HIGH HERITABILITY

• Average IQ (and height) have been steadily
  increasing in Western societies over the last 50
  years (and more).
• The overall genetic make up of populations in
  these societies does not change (much) over time.
• So, these changes must be environmental.
• How are these large environmental effects to be
  reconciled with the claim that, within a
  generation, IQ differences are largely genetic?

29
DICKENS AND FLYNN 2001

• Dickens and Flynn propose a model that allows
  large environmental effects, even with very high
  heritability (e.g. .75, which is at the upper
  end of the range of estimates for IQ
  heritability).
• Gene x Environment correlation
• analogy is someone who is (a bit) better than
  average at basketball
• they may be put into a (coaching) environment
  that makes them a lot better.
• The initial advantage in multiplied.

30
DICKENS AND FLYNN 2001

• However, initial disadvantage can also be
  multiplied.
• To account for a rise over time, there must also
  be some relatively pervasive factor that causes a
  rise over time
• With height it is better nutrition and better
  health generally
• Basketball analogy - television caused the rise
  in popularity of basketball over baseball because
  it was more suited to the small screen and
  increased its audiences more.
• More people play basketball
• Skills of professionals are copied
• General level of basketball skill increase over
  time

31
DICKENS AND FLYNN 2001

• Extending the analogy to IQ changes over time
  means recognising
• Gene x environment interactions and multiplier
  effects
• Some persistent environmental factors favouring
  ability to carry out cognitively complex tasks
• Maybe, more complex jobs
• Maybe, more leisure time with intellectually
  demanding leisure pursuits
• Maybe, smaller families allow parents to
  encourage intellectual development of children
• etc., etc.