The slogan “Breast is Best” has been popularized by medical organizations and parenting networks to extoll the benefits of breastfeeding, yet the causal effects are widely debated. Our study contributes to the debate by examining whether breastfeeding has differential effects based on the propensity to breastfeed, which is also known as causal effect heterogeneity. Prior studies attempt to isolate the causal effect of breastfeeding by netting out confounding characteristics, but we argue that the effects of breastmilk are unlikely to operate in a vacuum. The social forces that promote or constrain breastfeeding among different populations in American society can also shape its effects. Using rich intergenerational panel data from the NLSY79 Child and Young Adult cohort (n = 7902), we evaluate heterogeneous treatment effects in the relationship between breastfeeding and child development from ages 4 to 14 using stratification-multilevel propensity score models. We find that breastfeeding is associated with small benefits for behavioral development, math scores, and academic ability among those with the highest propensities to breastfeed. By contrast, its small benefits for reading comprehension and vocabulary are concentrated among children with the lowest propensities to breastfeed. Our findings suggest that the social process of selection into breastfeeding cannot be fully disentangled from its estimated effects. The social context not only shapes who breastfeeds in American society, but also who benefits most.

中文翻译：

---

最适合谁？母乳喂养对儿童发育的异质治疗效果


“母乳是最好的”这一口号已被医疗组织和育儿网络广泛宣传，以颂扬母乳喂养的好处，但其因果影响却受到广泛争论。我们的研究通过检查母乳喂养是否会根据母乳喂养的倾向产生不同的影响（也称为因果效应异质性），为这场争论做出了贡献。先前的研究试图通过消除混杂的特征来分离母乳喂养的因果效应，但我们认为母乳的影响不太可能在真空中发挥作用。在美国社会不同人群中促进或限制母乳喂养的社会力量也会影响母乳喂养的影响。利用来自 NLSY79 儿童和年轻人队列 (n = 7902) 的丰富代际面板数据，我们使用分层多级倾向评分模型评估母乳喂养与 4 至 14 岁儿童发育之间关系的异质治疗效果。我们发现，对于母乳喂养倾向最高的人来说，母乳喂养对行为发展、数学成绩和学术能力有微小的好处。相比之下，它对阅读理解和词汇量的微小好处集中在母乳喂养倾向最低的儿童身上。我们的研究结果表明，选择母乳喂养的社会过程不能完全与其估计的影响脱钩。社会背景不仅决定了美国社会中谁进行母乳喂养，还决定了谁受益最大。