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Susan Farber, a psychodynamically trained, biologically oriented psychologist, has gathered together a vast body of disparate reports on identical twins who were reared apart. She has collated the data into tabular form so that the reader can scan them quickly and use them to examine the nature/nurture interaction question in the developmental process. She has looked into a list of different items ranging from anthropomorphic traits and physical disorders to psychosis, IQ scores, and personality.
In an attempt to determine what appears to be stable in separated, monozygotic twins, the author has re-examined the twin data to see what can be viewed as reflecting either genetic predisposition or environmental influence in explaining the appearance of a trait. She has also considered the area of interaction between the two, so often neglected by either "geneticists" or "environmentalists."
Perhaps because the data summarized are not her own, Farber shows few obvious biases. She states:
Generalization from this sample of twins, reared apart, is open to debate. Most sets were chosen because they were highly similar in the first place, a fact that immediately eliminates them as a random-sample of twins reared apart. Additionally, generalization to the population of non-twins is limited by the bias in the sample toward low SES, prematurity, mental and physical illness, predominantly British and European rearing environments (most dating back to the turn of the century), war (all but a small fraction of countries occupied or under bombardment during World Wars), fluctuating degrees of separation, and outdated and questionable tests. Almost all are adult and Caucasian, and only 3 are genuinely reared-apart in the true meaning of the term (p. 170).