INTELLIGENCE, THINKING AND PERSONALITY

Genetic and Environmental Factors in Intelligence

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.

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.

THE HEREDITARIAN LEGACY - “THE BELL CURVE”

Arguments similar to Jensen’s 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 Jensen’s

“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

PHENOTYPE and GENOTYPE

Physical and psychological characteristics are part of an animal’s or person’s 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

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

HERITABILITY - cont

There is an issue of what constitutes the “group”.

But in any case, heritability is not a measure of between-group differences.

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.

“NATURE” versus “NURTURE”TWO 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

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

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.

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)

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.

TYPICAL RESULTS

Correlations of IQs of:1 MZ twins reared together 0.852 MZ twins reared apart 0.763 DZ twins reared together 0.604 Siblings, or parents and children 0.5

1 vs 3 suggests a genetic component 1 vs 2 suggests an environmental

component

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

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

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

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

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?

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

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 doesn’t 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)

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).

LEWONTIN’S 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?

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.

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

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

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?

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.

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

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.