Even when environmental factors were controlled by studying identical twins reared apart, the concordance rate remained the same.

The key to these statistics is the 'concordance rate', the proportion of twins in which both individuals suffer from the illness.

Instead, the concordance rate has been found to lie between 35 and 58 per cent.

Huntington's disease, for example, has a concordance rate of 100%, indicating a predominately genetic etiology.

However, when the concordance rate is lower, this indicates environmental factors like infectious microbes or toxin exposure are playing a causal role.

Controversial uses of twin data have looked at concordance rates for homosexuality and intelligence.

Thus, the concordance rate of a given characteristic helps establish whether or to what extent it is caused by genetic mutation.

There was a high concordance rate of 401/403 for pregnancies at 7 weeks gestation or greater.

The monozygotic concordance rate for the disorder is 70%.

However, female twin studies demonstrate that females have much lower concordance rates than males.