What Do Books in the Home Proxy For? A Cautionary Tale

In studies of educational achievement, students' self-reported number of books in the family home is a frequently used proxy for social, cultural, and economic background. Absent hard evidence about what this variable captures or how well, its use has been motivated by strong associations with student outcomes. I show that these associations rest on two types of endogeneity: low achievers accrue fewer books, and are also prone to underestimate their number. The conclusion is substantiated both by comparing reports by students and their parents, and by the fact that girls report on average higher numbers despite being similar to boys on other measures of social background. The endogenous bias is large enough to overturn classical attenuation bias; it distorts cross-country patterns and invalidates many common study designs. These findings serve as a caution against overreliance on standard regression assumptions and contribute to ongoing debates about the empirical robustness of social science.

在教育成就的研究中，家庭成员中自报的书籍数量常被用作社会、文化和经济背景的常用代理指标。鉴于缺乏关于该变量所捕捉内容及其准确性的硬性证据，其使用主要基于与学生成果之间强烈的关联性。本研究揭示，这些关联性建立在两种类型的内生性之上：成绩较低的学生拥有较少的书籍，并且也倾向于低估其数量。这一结论通过比较学生及其父母的报告，以及尽管在社会背景的其他衡量指标上与男孩相似，女孩平均报告的书籍数量更高的现象得到证实。内生偏差之大足以颠覆经典衰减偏差；它扭曲了跨国的模式，并使许多常见的研究设计失效。这些发现对过度依赖标准回归假设提出了警示，并为关于社会科学经验稳健性的持续辩论做出了贡献。