Biological and Social Aspects of Intelligence

It is clear from a range of studies of familial influences on IQ that a large number of genetic factors contribute to IQ. A common starting point for genetic analysis is linkage analysis within families. Since this requires an identifiable marker linked to the gene of interest, it can only be extended to whole populations if the genome is so densely mapped that very close markers indeed can be found. If recombination between gene of interest and marker is as little as 0.001 then linkage disequilibrium can persist for around 8000 years.

This approach assumes that an apparently continuous trait such as intelligence can be the result of a finite number of discrete genes acting together. Diagram 1 shows how as few as 3 loci giving rise to 7 phenotypes produce continuous variation within the population. This model has been shown to be valid for a wide range of characteristics from the quantity of juice in tomatoes to glucose tolerance in mice.

Should research into the molecular genetics of intelligence be done? It certainly will be done and it is therefore best done by those who recognise its limitations, its potential for misuse and the fact that the predictive power of its results are modest.

Mild mental retardation is characterised by an IQ in the range 50-69. Given that this represents 2-3% of the population, it is surprising that there has been no systematic study of genetic influences. It is therefore possible to draw few conclusions but instead to raise a series of vital questions.

An IQ of 50-69 may simply be the lower end of the normal range of variation or it may represent some specific pathology, genetic or otherwise. The most common specific causes are:

• Fragile X anomalies

• Other sex chromosome anomalies

• Prader-Will syndrome (microdeletion on chromosome 15)

• Williams syndrome

• Autism

• Cerebral palsy

• Epilepsy

• Obstetric/perinatal complications

There is a pressing need for a carefully designed, wide-ranging research programme. This should include:

• An epidemiological survey of children in the 50-70 IQ range

• Study of monozygotic twin pairs discordant for mild retardation

• Consideration of why the social impairment and needs of people in this IQ range vary so much

• Investigation of the connection between socially disadvantaged backgrounds and mild retardation.

The considerations when assessing the adequacy of psychogenetic methods include:

• adequacy of the model as a representation of reality

• appropriateness of the experimental material as representing the population of interest

• adequacy of experimental design

There is seldom any problem modelling a trait involving a single gene. The treatment of complex, continuously variable traits was tackled by Fisher in 1918 on the assumption that there are a number of genes each with an equivalent small additive effect. This model has proved very succesful with agricultural crops and some animals. Substantial progress is now being made with the identification of such polygenes in plants using markers.

Is such a simplistic model a plausible representation of the genetics of human traits such as intelligence? Is it likely that the effects of genes will be additive both amongst themselves and with environmental effects? If the number of separate effects is large and - as is a necessary consequence - the size of each is small, even non-linear effects will behave linearly.

So, the model is plausible but the real test is to apply it and see if it produces plausible results. When applied to the question of whether the genetic component of intelligence varies with age, useful patterns emerge. We can expect this sort of approach (known as "QTL") to dramatically change the way geneticists think about behaviour.

Galton first recognised the need to separate out the environmental and genetic components that are confounded within families and proposed the study of adopted children. He also recognised the importance of twins, having a theory of their etiology that proved much more accurate than Darwin’s, but apparently never proposed studying twins reared apart.

It required the subsequent development of standardised IQ tests to provide scientific, rather than anecdotal, evidence of the genetic basis of intelligence. In the 1960s it was still possible to plausibly argue the environmental case but since then more and more good data has accumulated and the scientific case is now proven. Five good studies now agree on a high genetic component for intelligence with a correlation of 0.75. All that more data will do is add decimal places to this figure. The onus is clearly on the environmentalists to show why all these studies are flawed and this they have failed to do. So why is the principle still disputed, why is the argument interminable?

The fact is that everyday life is unscientific; culture is supported by many influences other than science. Science never answers all the questions, so one can always argue that there is not enough scientific knowledge on a particular subject.

Modern studies are showing that different genes act at different ages and that the environmental effects that can influence IQ in childhood dissipate with age so that one’s intelligence in later life is largely determined by one’s genes. The nature of intelligence is now a subject of hot debate - is it just an aggregation of particular skills or is there a general intelligence, g, that pervades all aspects of an individual’s cognitive performance?

It is sometimes argued that the sexes score equally on IQ tests because that is the way the tests are designed and IQ is defined. This is a common but incorrect assertion. The early IQ tests were thought (by Burt et al) to provide the means of answering the question "Are the sexes equal?" and to have shown that indeed they are. There were differences on some types of test, but the more a test correlated with general intelligence, the more equally the sexes scored. It is however true that in designing later batteries of tests, tests showing large sex differences were eliminated as probably unfair.

Were these early conclusions right? Although overall the scores of the two sexes were similar, individual tests showed considerable differences in favour of one sex or the other that just happened to cancel out. Recent tests show considerable difference, depending on the exact mix of tests. What this really questions is the scientific value of overall IQ as measured by a battery of tests.

But the differences on particular tests are real and men really win out on mechanical reasoning and spacial orientation. The reasons for these differences might cast light on brain function, but so far little research has been done. Another common argument is that males show greater variability so that there are more very clever and very stupid men than there are such women. This, too, seems unsupportable on the evidence.

If one group, say an ethnic group, is disadvantaged in some way, for example because on average its members have lower IQs, what should be done about it? If one takes the view that individuals in society who are unlucky should be compensated, then the bad luck of being born a member of a disadvantaged group presumably merits compensation. If there are objective differences between ethnic groups, discrimination is inevitable even in the absence of racial prejudice because the effort to identify exceptions cannot be justified - for example by banks screening loan applications.

Is not a meritocracy possible, in which rewards are allocated according to merit? In order to achieve this it would be necessary to allocate rewards irrespective of merit to equalise environments. So a meritocracy is impossible.

Thus, argued Flynn, the case for affirmative action for US blacks appears to be proven.

The nature/nurture debate in respect of intelligence began with Galton who emphasised the role of nature. The pendulum has swung back and forth since then with modern research indicating a heritability for IQ of around 50%, more in later life.

It is interesting that not only are both spacial and verbal abilities highly heritable, but their covariance indicates that the same genes contribute to both.

The contributions to variance in IQ in childhood can be visualised in diagram A. In later life, however, the relationship is as shown in diagram B - the contribution of the shared environment has gone.

By way of illustration, adoptive siblings correlate for IQ at the age of 8 but after childhood that correlation diminishes to zero.