Why do I bring this up? Because I have recently found myself wondering what exactly has happened to democratic peace theory (DPT). As students of international relations all know, DPT was a major intellectual preoccupation for IR scholars from the mid-1980s until well into the 21st century. Beginning with Michael Doyle’s seminal works on the topic (refining arguments originally developed by Immanuel Kant ), the idea that “democracies don’t fight each other” inspired a vast outpouring of scholarly articles and books, along with a number of important critiques.

Some social science theories have remarkable staying power; others turn out to have a more limited shelf life. Sometimes a once-promising idea just doesn’t seem to lead anywhere—Talcott Parsons’s structural-functionalist approach to sociology might be one example—and eventually most scholars abandon it and move on. Or a novel theoretical argument can sound compelling when it first appears, but subsequent research reveals its logical or empirical limitations. In other cases, the real world delivers a harsh verdict on a bold claim—remember the “end of history” thesis?—although some discredited theories can survive, zombie-like, because powerful interests find it useful to keep them alive.

Some social science theories have remarkable staying power; others turn out to have a more limited shelf life.