摘要：Objectives. This study sought to estimate the impact on birthweight of maternal participation in the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC). Methods. WIC estimates were based on sibling models incorporating data on children born between 1990 and 1996 to women taking part in the National Longitudinal Survey of Youth. Results. Fixed-effects estimates indicated that prenatal WIC participation was associated with a 0.075 unit difference (95% confidence interval [CI] = –0.007, 0.157) in siblings' logged birthweight. At the 88-oz (2464-g) low-birthweight cutoff, this difference translated into an estimated impact of 6.6 oz (184.8 g). Conclusion. Earlier WIC impact estimates may have been biased by unmeasured characteristics affecting both program participation and birth outcomes. Our approach controlled for such biases and revealed a significant positive association between WIC participation and birthweight. Established in 1972, the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) attempts to increase the nutrition levels and general well-being of children. 1, 2 Many of the evaluations of this program were conducted at least 10 years ago, and the majority have relied on data from only a few states. 3– 8 Previous studies of the effects of WIC participation on children's well-being involved a risk of bias resulting from unmeasured selection factors. Not all eligible women claim benefits, and if selection into WIC involves the neediest women, studies comparing child outcomes among WIC participants and nonparticipants that fail to adjust for differences in “need” may underestimate the effects of the program. In contrast, if WIC participants are more highly motivated than nonparticipants, then studies that fail to adjust for motivation may overestimate the program's effects. Because the WIC program is locally administered, factors governing selection into the program are also likely to vary across time and place. In the present study, we addressed these concerns by estimating WIC effects with a national sample of children and using sibling fixed-effects models to account for unmeasured heterogeneity among the mothers of sample children. Specifically, we studied a sample of children born between 1990 and 1996 to mothers participating in the National Longitudinal Survey of Youth (NLSY), and we used merged NLSY mother–child data files to estimate the effects of WIC participation on birthweight. We eliminated the biasing effects of persistent characteristics of mothers—both measurable and unmeasurable—and thus the estimates from this study constitute a methodological advance over previous studies. A disadvantage of the sample is that all of the children were born to somewhat older mothers. In addition, all of the data were reported by the mothers, and standard errors for the fixed-effects models that we estimated were considerably larger than standard errors for more conventional regression models.
