structures and feedback properties, expose them to perturbations, observe their recovery, and then—in the same way that one might “train” a chess-playing program—modify these systems until they become more tolerant of the disturbances to which they are exposed. Doyle and Carlson’s strategy offers a way to improve the structure of systems when the mathematics cannot be solved. Nevertheless, as the authors themselves point out, their approach does have a drawback: Systems that are engineered or have evolved to be tolerant of a particular set of disturbances often do so at the expense of their response to other classes of disturbances. Such systems are at once robust and fragile—an outcome that policymakers and researchers might wish to guard against as they seek better ways to manage risk and avert systemic failures.12
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