Environmental versus genetic interpretations of birth‐order effects

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Abstract
Since the effects of birth-order differences on the risks of pre- and post-natal morbidity and mortality, like those of parental age differences, may reflect to some degree the operation of genetic causes, one must consider in what ways such causes may be distinguished from those which are environmental in origin. Data indicate that early mortality, whether in the form of stillbirths, infant deaths, or deaths beyond the first year of life, becomes an increasing risk with advancing pregnancy order or birth order, independent of the effects of the mother''s age. A distinction may be made between the large birth-order effect for deaths beyond the first week of life, which is probably environmental, and an effect for deaths in the first week of life, the causes of which are not known. An apparent absence of birth-order effect in the risks of handicaps of children, indicates that such mechanisms play a minor role in the production of such traits.