Compulsory Education and the Benefits of Schooling
利用美国各州义务教育法作为工具变量，研究发现允许出生年份效应随地区变化后，原本显著的因果估计（如对工资、失业和离婚的影响）变得不显著甚至符号相反，质疑了共同趋势假设的可靠性。
Causal estimates of the benefits of increased schooling using US state schooling laws as instruments typically rely on specifications which assume common trends across states in the factors affecting different birth cohorts. Differential changes across states during this period, such as relative school quality improvements, suggest that this assumption may fail to hold. Across a number of outcomes including wages, unemployment, and divorce, we find that statistically significant causal estimates become insignificant and, in many instances, wrong-signed when allowing year of birth effects to vary across regions.