Changes in sibling configurations for American preschool children.

This paper uses data drawn from the 1940 through 1980 Public Use Microdata Samples of the U.S. Census of Population to document sibling configurations from the child's perspective. Changes in four aspects of siblings are examined for five cohorts of white and black preschool-aged children: number, birth order distributions, spacing intervals, and sex composition. Changes in fertility behavior of adults in the post-war era had a profound effect on the structure of sibling systems experienced by children. Successive cohorts of preschool children show a rise in number of siblings through the early post-war years before showing sharp declines in number of siblings through the 1960's and 1970's. These shifts in size of sibling sets are reflected in changes in the proportion of each cohorts who are first born and only children, both of which have increased substantially by the 1980 cohort. The 1940 and 1980 cohorts have similar proportions of children with short intervals. However, the middle cohorts show the effects of the quickened pace of fertility with substantial proportions of children with comparatively short birth intervals. Finally, substantial shifts across cohorts in several measures of sex composition of children are observed. Most significantly, there is a marked decline in the proportion of children experiencing an opposite-sex older sibling.