But a different situation obtains if I shoot not by calculation but by nervousness. Suppose that my nervousness depends on how frightened I am, and my fright depends on how likely I think it that he may shoot me; and suppose he acts the same way. Then when I consider the exogenous probability that he may shoot me out of sheer preference, it makes me nervous; this nervousness enhances the likelihood that I may shoot him even though I prefer not to. He sees my nervousness and gets nervous himself; that scares me more, and I am even more likely to shoot. He sees this increment in my nervousness, and matches it with one of his own, scaring me further; and the probability that I will shoot goes up again. Now we can denote each person's nervousness as a function of the other's, and the likelihood of shooting as a function of nervousness, and have a simple pair of simultaneous differential equations that seem to yield precisely the kind of phenomenon we started off to study.

XREF: Connects to escalation dynamics in game theory, arms races, and conflict spirals—the classic prisoner's dilemma and security dilemma literature.

Thomas C. Schelling, The Strategy of Conflict, loc. 485