Sibship Size and Intellectual Development: Is the Relationship Causal?

In this study we test the alternative interpretation of the effect of sibship size on childs intellectual development through sibling analysis and analysis of repeated measures of the same individuals. Both analyses are variations of change models or fixed-effects models. Change models enable us to control permanent family effects including family socioeconomic status (SES) family genetic makeup and intellectual atmosphere in the home by `differencing them out. Thus we can determine if and how much the sibship-size effect is confounded by other family influences that are difficult or impossible to control in conventional regression analysis. The data are from the [U.S.] National Longitudinal Survey of Youth (NLSY). Also included are comments by Meredith Phillips (pp. 188-92) and D. B. Downey B. Powell L. C. Steelman and S. Pribesh (pp. 193-98) and a reply by Guo and VanWey (pp. 199-206). (EXCERPT)