Eysenck and Prell's 1951 study of 25 identical and 25 fraternal pairs of twins concluded that the neurotic personality factor was a biological; they estimated the genetic contributions at 80 percent. Based on Eysenck and Prell's report of the statistical procedures used, the authors believe Eysenck and Prell's interpretation should be rejected or severely limited; a methodological analysis was undertaken. The Eysenck-Prell study derived a neuroticism factor from factor analysis of a battery of 17 tests. Although they found identical and fraternal twins to have almost identical means with respect to neuroticism, the identical group contained pairs which were more extremely unstable and more extremely stable than the fraternal group. Using Holzinger's statistical technique, with reservations, they found that identical twins showed a correlation on neuroticism of .851, while fraternal twins showed .217. They concluded that individual differences in neuroticism were very markedly determined by heredity and very much less so by environment. A number of problems were noted with their methodology: the sample size was reduced; the computation of h 2 was faulty since the variance of the two groups was not equal; correcting the raw intraclass correlations was less useful than using comparable groups in the first place; results differed when the reference group was fraternal rather than identical; there were problems with the h 2 statistic; environmental variance was not analyzed; and the validity of the neuroticism criterion was dubious. It would have been most fruitful to study the interaction of genetic and environmental factors.