The Depression-prone and the Depression-resistant Sibling: A Study of 650 Three-Sibling Families

Abstract

The relationship between ordinal position of birth and mental illness, and particularly schizophrenia, is a familiar issue to regular readers of this journal. Its significance rests mainly in the fact that on genetic grounds each ordinal position of birth in a sibship is equally vulnerable. Therefore conclusive evidence of a positive relationship between birth order and mental illness, while not excluding the importance of hereditary factors, offers a good indication of the operation of some, possibly early, familial environmental influences.