Smoking Habits of Monozygotic and Dizygotic Twins

Various epidemiological investigations?for example, Doll and Hill (1954, 1956), and Hammond and Horn (1954)?have, independently of each other, demonstrated higher morbidity and mortality from, inter alia, cardiac disease and pulmonary cancer in tobacco smokers as compared with non-smokers. These findings have often been cited as evidence of a direct causal association between smoking and the diseases in question. The planning of the inquiries and the interpretation of results, however, have been sharply criticized from several quarters. For instance, it has been pointed out that in many respects smokers differ essentially from non-smokers, and for this reason comparison between the two groups is not entirely warrantable. An important source of error in comparing smokers and non-smokers with respect to morbidity and mortality lies in the possibility of constitutional dissimilarities between the groups. It is thus conceivable that a common factor predisposes to the diseases as well as to excessive smoking. A study of a sufficient number of monozygotic twin pairs who differ in their smoking habits should substantially eliminate such sources of error and show the possibility of a causal connexion between smoking and disease. Before an investigation of morbidity and mortality can be embarked upon, therefore, the smoking habits of twins must be charted. An inquiry of this type should also indicate the importance of the genotype in relation to smoking habits. If, for instance, it is found that twins of monozygotic pairs are more alike in smoking habits than twins of dizygotic pairs, the suggestion is that constitutional factors may significantly influence these habits.