An investigation of family size and birth order as risk factors in leprosy.

To investigate if early exposure to Mycobacterium leprae is a factor determining development of Hansen's disease in general, or of a particular form of the disease, we have studied the sibship size and birth order distribution of 187 leprosy cases and 528 hospitalized control patients. By the Greenwood-Yule analysis, a small deficit of cases in the earlier birth order in the tuberculoid form was observed. Such a deficit was not observed in lepromatous leprosy patients. In the multivariate analysis, where socioeconomic factors, current age, and leprosy status of the parents were taken into account, it was found that sibship size becomes a nonsignificant and nonconsistent predictor of leprosy in general or of a particular form of the disease. On the other hand, the logistic analysis supports the impression that tuberculoid leprosy patients belong to the later birth order in both sexes. Although the contrast is statistically nonsignificant in either sex, it suggests that early exposure in life predisposes to the tuberculoid form of leprosy.