General regularities, whether in the natural or the social sciences, have "scope conditions." They accurately describe the world while certain background conditions — in this case, air pressure — hold. When those background conditions change, the supposed laws have to be refined, circumscribed, or jettisoned.
This point is of course well known to social scientists. But while we fully understand the importance of scope conditions in the abstract, we rarely think seriously about how they might invalidate a lot of our key assumptions — and systematically blind us to history’s rare yet incredibly important turning points.
Consider these hypotheses: Senior members of American political parties get to pick their nominees. Extreme candidates who attack basic democratic norms do not gain mass support among members of the American (or Polish or Swedish) public. If a wealthy country has experienced at least two turnovers of government by free and fair elections, liberal democracy is there to stay. Each of these has been confirmed by subtle and extensive empirical evidence drawn from the last 50 years. And yet all of these hypotheses might be subject to scope conditions that are invisible to our eyes because they have held true for the last half-century — and are now giving way.
Do parties still retain the same influence over primary voters once public trust in politicians has plummeted to record lows? Do voters still shun extremist candidates at a time when they are less and less invested in the idea of democracy? And will wealthy liberal democracies continue to be stable when, for the first time since their founding, the living standards of average citizens have barely increased in a generation?
Natural scientists can, to some degree, test the scope conditions of their findings in a systematic manner. If there is some theoretical reason to suspect that air pressure might influence the boiling point of water, and if it is impractical to carry out experiments atop Mount Everest, they can artificially lower the air pressure in their laboratory. Political scientists have far fewer tools at their disposal. Even if they realize that there is some theoretical reason to suspect that liberal democracy might prove less stable when living standards no longer rise, they cannot run an artificial experiment that confirms the scope conditions of their theory.
The obvious takeaway is that political scientists need to be less confident about the general applicability of their findings. The less-obvious takeaway is even more radical: In a field in which we can never systematically determine the scope conditions of our theories, the discovery of general regularities may have less value than meets the eye. To discover that a high GDP boosts the stability of liberal democracy under some unknowable set of scope conditions, for example, is ultimately not very helpful: It neither gives a full explanation for why democracy has historically been firmly established in the United States, nor does it tell us under what circumstances American democracy might fail.