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In a seminal contribution Abel, Mankiw, Summers, and Zeckhauser (1989) show that from an aggregate dynamic perspective the US economy is Pareto efficient. We argue that, when applying their test, they implicitly make strong assumptions about the economy's future behavior. We show how time series evidence may easily lead to wrong conclusions about the welfare properties of real world economies.We present a test criterion based on Zilcha (1991) and robust evidence that the US economy does not overaccumulate capital. Our contribution highlights that the distinction between efficient capital accumulation and Pareto efficiency is empirically relevant. The latter efficiency benchmark - encompassing also risk sharing issues - cannot be rigorously tested based on available approaches in the literature.

Recently, Abadie et al. (Am J Polit Sci 59:495–510, 2015) have expanded synthetic control methods by the so-called cross-validation technique. We find that their results are not being reproduced when alternative software packages are used or when the variables’ ordering within the dataset is changed. We show that this failure stems from the cross-validation technique relying on non-uniquely defined predictor weights. While the amount of the resulting ambiguity is negligible for the main application of Abadie et al. (Am J Polit Sci 59:495–510, 2015), we find it to be substantial for several of their robustness analyses. Applying well-defined, standard synthetic control methods reveals that the authors’ results are particularly driven by a specific control country, the USA.

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