Here’s a great quote from Pearl and Bareinboim (2014, p.2) [link] in their analysis of “external validity” and conditions that allow for one to transport the results of a causal analysis from one context to another:
[The literature on external validity] consists primarily of threats, namely, explanations of what may go wrong when we try to transport results from one study to another while ignoring their differences. Rarely do we find an analysis of “licensing assumptions,” namely, formal conditions under which the transport of results across differing environments or populations is licensed from first principles.
The reasons for this asymmetry are several. First, threats are safer to cite than assumptions. He who cites “threats” appears prudent, cautious and thoughtful, whereas he who seeks licensing assumptions risks suspicions of attempting to endorse those assumptions.
Second, assumptions are self destructive in their honesty. The more explicit the assumption, the more criticism it invites, for it tends to trigger a richer space of alternative scenarios in which the assumption may fail. Researchers prefer therefore to declare threats in public and make assumptions in private.
Third, whereas threats can be communicated in plain English, supported by anecdotal pointers to familiar experiences, assumptions require a formal language within which the notion “environment” (or “population”) is given precise characterization, and differences among environments can be encoded and analyzed.
There are so many truths in there that extend beyond research on external validity.