A mathematical model is proposed, based on catastrophe theory, to describe the qualitative effect of stress upon the neural mechanisms used for making judgements, such as estimating speed. The model is used quantitatively to fit data, and to explain the cusp‐shaped results of Drewet al.(1959), showing that introverts under alcohol tend to drive either too fast or too slow in a driving simulator. Experiments are suggested in which discontinuous jumps in perception of continuous variables like speed might well appea