Familial tendencies in diseases of children.

The clustering of cases of disease within family groups has long been of interest to geneticists, but the empirical data which they have gathered concerning the strengths of these associations have had to do largely with conditions that were thought in advance to be either hereditary or at least partially so. Epidemiologists, who might be expected to be interested in the strengths of familial predispositions other than those due to heredity, have perhaps lacked the specific motivation of the geneticists and have certainly been hampered as well by a lack of appropriate methods for quantitative study of the problem on any substantial scale. Conventional sampling techniques, for example, tend to be unsuitable for studies of recurrences of similar events of ill health among different members of the same families, and are especially so where the diseases in question are rare. Although this particular obstacle to investigation of the familial component of ill health might be avoided by the use of routine records of illnesses and of family compositions covering whole populations, such records have not in the past been organized in a manner that would facilitate quantitative studies of the special risks to relatives of affected individuals. Methods are now available, however, for obtaining family histories of procreation and of mortality and morbidity from the routine records of vital and health events, throughout whole registration areas (Kennedy, 1961, 1962; Kennedy, Newcombe, Okazaki, and Smith, 1965; Newcombe and Kennedy, 1962; Newcombe, Kennedy, Axford, and James, 1959). Such methods have been used in the present study to measure the strengths of the family associations of a wide range of ailments, including many that are caused mainly by environmental factors or for which there is currently little evidence of an hereditary component in the aetiology.