A shared family environment for twins reared together is assumed.

However, some studies of twins reared apart (e.g. Bouchard, 1990) find a significant shared environmental influence, of at least 10% going into late adulthood.

The biggest surprise to come from the Bernard research is that twins reared apart aren't any more different than those reared together.

He believed that hereditary factors accounted for 85 percent of intelligence and developed a study of twins reared apart to prove it.

A study of twins reared apart has come up with one of the highest estimates yet for the heritability of intelligence, as judged by test scores.

Bouchard's longitudinal studies of twins reared apart are world-renowned.

Moreover, the concordance data on twins reared together do not distinguish conclusively between genetic and environmental transmission.

A limit of this design is the common prenatal environment and the relatively low numbers of twins reared apart.

Studies of twins reared together or apart have tried to disentangle the competing influences of nature and nurture on everything from intelligence to schizophrenia.

The book also includes research on twins reared apart and information on several other switched at birth twin cases.