Expressing inheritance
When psychologists explore the extent to which a characteristic (e.g., intelligence,
schizophrenia) is inherited, they use various different techniques.
The concordance rate expresses the extent to which two measures are in
agreement. For example, suppose that 20 twin pairs are studied in which at least
one twin has developed schizophrenia. If there are 18 pairs in which both twins
have schizophrenia, then the concordance rate is 18/20 or 90%, indicating a very
high level of concordance. We can take things further if we compare concordance
rates in monozygotic (identical) twins and dizygotic (non-identical or fraternal)
twins.
Monozygotic twins share 100% of their genes, whereas dizygotic twins share only
50%. If genetic factors are important, we would expect monozygotic twins to have a
higher concordance rate than dizygotic twins. If genetic factors don’t play a role,
then there should be no difference in concordance rate between the two types of
twins.
We can use information from concordance rates to work out heritability. This
provides an estimate of the extent to which genetic factors contribute to individual
differences in some characteristic within a given population. Heritability of 1.0
would mean that all the variance or variation within a given population was due to
genetic factors, and heritability of 0.0 would mean that genetic factors were totally
irrelevant.
We can use information about IQ from pairs of monozygotic and dizygotic twins in a
similar way. In essence, if individual differences in intelligence depend to some
extent on genetic factors, then identical twins should resemble each other more in
IQ than should fraternal twins. If environment is all important, then identical twins
should be no more similar to each other in IQ than fraternal twins. In either case, the
outcome of taking account of the relevant information is an estimate of heritability
of intelligence.