ENVIRONMENTAL VERSUS GENETIC INTERPRETATIONS OF BIRTH-ORDER EFFECTS.

Abstract

The effects of birth-order differences on the risks of pre and postnatal morbidity and mortality like those of parental age differences may reflect to some degree the operation of genetic causes. This being the case there is reason to consider in what ways such causes may be distinguished collectively from those that are environmental in origin and to what extent present information may serve to place limits on the magnitude of the genetic component. Use has been made of information contained in routine records of live births stillbirths child deaths and handicaps of children in British Columbia in order to interpret the influences of birth-order effects. It was found that early mortality whether in the form of stillbirths infant deaths or deaths beyond the first year of life becomes in general an increasing risk with advancing pregnancy order or birth order independent of the effects of the mothers age. A distinction may be made between the large birth-order effect for deaths beyond the first week of life which is almost certainly environmental in origin and a similar but smaller effect of still-births and deaths in the first week of life the causes of which are not known. It is noted that maternal-fetal incompatibilities that give rise to progressively increasing risks of sensitization of the mother with each successive pregnancy might perhaps contribute substantially to the latter but not to the former. An apparent absence of a birth-order effect in the risks of registerable handicaps of children as judged from the combined data for all causes indicates that such mechanisms if they are involved at all in the production of these traits must play a minor role. A few conditions notably strabismus and certain other congenital malformations of the nervous system and sense organs for which birth-order effects have been observed independent of mothers age effects may be exceptional in this respect.