Beyond Sibship Size: Sibling Density, Sex Composition, and Educational Outcomes

We explore the effects of sibship density, i.e., the number of siblings closely spaced versus widely spaced, and sex composition, i.e., the number of sisters versus the number of brothers. Guided by two explanations previously put forth to account for the consistently found inverse relationship between sibship size and academic outcomes, the confluence model and the resource dilution hypothesis, we test whether these two structural parameters influence academic consequences. Analysis of the High School and Beyond data confirms a stronger negative effect of the number of siblings closely spaced than of the number of siblings widely spaced, an effect thiat endures into later adolescence even upon controlling for earlier test performance. Findings on sex composition, based on the National Longitudinal Survey of Highi Schlool Seniors of the Class of 1972, are mixed. Wefind no effect of sex comnposition on test performance, but find a negative one on grade-point average. We conclude by hlighilighltinig thie utility of decomposing sibship size into further coinponents and of obtaininig data thlat will facilitate such research efforts. Although the antecedents of fertility behavior have sustained long-standing interest among social demographers, research priorities have widened recently to encompass the consequences of differential fertility (Blake 1981). It is not only the question of which factors affect the number and timing of the children people have that intrigues social scientists but also the aftermath of varying sibship structures. The firmly documented inverse relationship between the number of -siblings and academic achievement, for example, has elicited considerable attention not only from demographers but also from sociologists with interests in education, social psychologists, status-attainment researchers, medical academicians, educators, and the public at large. While sociologists investigating educational processes view the determinants of ability and/or *This research was supported by NSF Grant SES-8508301 to Steelnan and a SpencerFellowship to Powell. The authors' names are listed in alphabetical order. Their contribultions are equLal. We appreciate assistancefromn Ann Smnith, Debra Brown, Donna Eder, Sutsan Engel, Carl Ek, and Christie Spruill. Commnentsfrom two anonymous reviewers are also gratefiully acknowledged. Please direct all correspondence to Lala Carr Steelinan, Departnent of Sociology, University of South Carolina, Columbia, South Carolina 29208. i) The University of North Carolina Press Social Forces, September 1990, 69(1):181-206 This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 06:00:22 UTC All use subject to http://about.jstor.org/terms 182 / Social Forces 69:1, September 1990 achievement as having value in their own right, others such as those elaborating models of status attainment see virtues in this topic because of the direct tie between ability/achievement and eventual educational and occupational attainment. Sewell and Hauser (1976) observe that a large component in the variance of the educational attainment of sons depends upon the independent effect of ability. Consequently, the number of siblings has become a standard inclusion in the usual set of status-attainment predictors (Blau & Duncan 1967; Featherman & Hauser 1978; Alwin & Thornton 1984; Sewell, Hauser & Wolf 1980; Blake 1985). Researchers, however, have failed to move forward by decomposing the size of the sibling group into its other theoretically meaningful axes. The research reported in this article augments previous theoretical and empirical endeavors on the consequences of the number of siblings by examining the effects of sibling density, i.e., the number of siblings within specific time intervals contiguous to the individual, and sex composition of the sibship, i.e., the number of brothers and the number of sisters, on academic performance. The objectives of this research are twofold: to underscore the merits of family density and sex composition as theoretically meaningful components of sibling structure and to provide substantive information on the effects of these variables on educational outcomes. Sibship Size and Academic Outcomes The body of research directed toward understanding the relationship between sibling configuration, intellectual growth, and actual educational performance continues to thrive. The usual thrust in this literature is aimed at the impact of the number of siblings, the effect of birth order, or the consequences of both. Although the contemporary trend has been to discount its significance, whether ordinal position in the family shapes an individual's level of ability remains a contested matter (Steelman 1985; Cicirelli 1978). Nonetheless, the negative relationship between the number of siblings and educational outcomes has received such consistent support that this pattem now is almost taken for granted (Steelman 1985; Cicirelli 1978; Anastasi 1956). Indeed this relationship is one of the firmest found in sociological research. It is from previous attempts to account for the family size/ability and achievement connection that the underpinnings for this work evolved. A variety of explanations for the deleterious impact of the number of siblings on educational performance has been put forth. While physiological and genetic reasons are sometimes advanced to account for this pattern (Grotevant, Scarr & Weinberg 1977; Bayer 1967), two environmentalist interpretations command the most acceptance in sociological circles. They are the confluence model and the resource dilution hypothesis.