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(1) This paper presents the results of a field study designed to investigate the hypothesis that persons presumed heterozygous for phenylketonuria are more vulnerable to mental disorder than other persons. Three-hundred-and-thirty-one persons were interviewed, including 108 parents of phenylketonuric offspring, 102 parents with non-phenylketonuric mentally retarded offspring, and 121 parents of children with cystic fibrosis. (2) Information was collected by means of a standard interview schedule which inquired into mental health problems of the interviewees as well as their parents and siblings. Interviews were coded according to an explicit code. Mental Health was evaluated by a set of indices constructed by assigning numerical values to certain items in the code and summing related items. These indices served as operational definitions of mental illness. (3) Analysis of variance was used to evaluate data derived. (4) Decreasing social class appeared to be associated with increasing scores on the indices. In addition, men and women scored significantly differently on many of the indices. In general, differences due to sex and social class were in the direction which would have been expected if the indices had been measuring the problem areas we were trying to evaluate. (5) The parents of phenylketonuric children did not score differently from the other two groups, indicating that they were not more susceptible to mental health problems than the controls, at least as measured by our operational definitions. (6) Some of the problems associated with field studies of psychiatric disease are discussed.

We have presented a series of constructs which were developed to measure the relative mental health of the members of three small populations. These constructs were highly concrete, and each of the 331 respondents investigated could be assigned a numerical rating without the use of clinical judgements of trained personnel. These constructs proved to be intercorrelated to a large degree. It was demonstrated that the size of the correlation between constructs was not always an accurate reflection of relationships within the population, so that certain relationships which were highly significant for men proved to be insignificant for women, and vice versa. The heterogeneous nature of the population could be shown to lead to two kinds of errors when data was looked at only for the population as a whole. (1) Loss or diminution of relationships which were true for one group but not the other; (2) errors in the interpretation of the significance of a relationship. It was pointed out that these errors probably represent general difficulties in the analysis of data from psychiatric studies.