Sibship Size and Mental Illness

Abstract

Though there have been many investigations concerned with sibling order and mental illness there have been relatively few concerned with sibship size. The abundance of sibling order studies is probably due to the widely accepted belief that in such studies comparison with general population data is not necessary. Expected numbers within each birth rank are estimated by the Greenwood-Yule method, proposed in 1914, by which it is assumed that for any given size of sibship siblings should be evenly distributed among birth ranks. In the present population, in which family size has in recent years been decreasing, this assumption is clearly invalid, for the younger members of the larger families of earlier years are more likely to be included in a contemporary sample, the older members by now having died. Other factors influencing sibling position have been more fully discussed by Price and Hare (1969). Thus comparison with general population figures is necessary in sibling order studies, and a report on such a comparison will be published separately by the author.