External validity captures the extent to which inferences drawn from a given study's sample apply to a broader population or other target populations. Social scientists frequently invoke external validity as an ideal, but they rarely attempt to make rigorous, credible external validity inferences. In recent years, methodologically oriented scholars have advanced a flurry of work on various components of external validity, and this article reviews and systematizes many of those insights. We first clarify the core conceptual dimensions of external validity and introduce a simple formalization that demonstrates why external validity matters so critically. We then organize disparate arguments about how to address external validity by advancing three evaluative criteria: model utility, scope plausibility, and specification credibility. We conclude with a practical aspiration that scholars supplement existing reporting standards to include routine discussion of external validity. It is our hope that these evaluation and reporting standards help rebalance scientific inquiry, such that the current obsession with causal inference is complemented with an equal interest in generalized knowledge.
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