20 October 2019

IQ and identical twins

The following extract is from: Peter Saunders, Social Mobility Myths, Civitas 2010, pp.56-58. (The full publication is available for download at civitas.org.uk.)
Given that intelligence is a function of both ‘nature’ and ‘nurture’, and that these two factors are each themselves entailed in the other, it is obviously extremely difficult to partial out their respective influences. But it is not impossible. Hans Eysenck claims that heredity is twice as important as environment in explaining differences in intelligence, and he bases this estimate on the results of repeated experiments carried out over many years by many different researchers. These experiments compare variations in mental ability between people who are unrelated genetically but who share a common environment (e.g. children raised in children’s homes) with variations between people who are genetically related but raised in contrasting environments (e.g. twins raised by different sets of foster parents). Many attempts have been made to discredit this work, but [Eysenck’s] overall conclusion is compelling and incontrovertible.

The strongest experiments focus on the performance of identical (monozygotic) twins as compared with non‐identical (dizygotic) twins. MZ twins share all their genes in common while DZ twins share 50 per cent of their genes. Ignoring Cyril Burt’s disputed findings, and aggregating the results of other researchers whose integrity has never been questioned, Eysenck reports the following average correlations in intelligence test scores:

• MZ twins raised in the same environment = 0.87
• MZ twins reared in separate environments = 0.77
• DZ twins raised in the same environment = 0.53

These figures compare with an average correlation of 0.23 for biologically unrelated individuals who are raised in a common environment (e.g. adopted or foster children), and with a correlation of zero for unrelated children raised in different environments. [...]

If environment were more important than heredity, the relative strength of these correlations should be reversed. Identical twins raised separately should differ more in their scores than non‐identical twins raised together, for they have been subjected to greater environmental variation. The opposite, however, holds true. Even when brought up separately, identical twins score much more similarly on IQ tests than non‐identical twins who were kept together. [...] To the extent that anything is ever proven in social science, the undisputed fact that identical twins brought up separately correlate so much more highly on test scores than non‐identical twins raised together proves that intelligence is based to a substantial degree (perhaps 50 per cent, probably more) on a cluster of genes which we inherit from our parents.
According to Professor Saunders, research on intelligence ‘has clearly demonstrated that we are not all born equal, despite the wishes of egalitarian sociologists that we were.’

Image source: Raul Carabeo.