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 parameters without serious negative consequences. The basic engineering principles underlying the creation of nuclear weapons exploit this feature of positive feedback mechanisms: the destructive output of nuclear weapons results from the energy released during the fission of certain isotopes of (in most cases) uranium or plutonium. Since fission of these heavy isotopes produces (among other things) the high-energy neutrons necessary to begin the fission process in other nearby atoms of the same isotope, the fission reaction (once begun) can—given the right conditions—become a self-sustaining chain reaction, where the result of each step in the cycle causes subsequent steps, which are both similar and amplified. Once the fission reaction begins it reinforces itself, resulting in the rapid release of energy that is the nominal purpose of nuclear weapons.

Of course, in most real-world cases the parameters involved in positive feedback loops are not able to increase without bound. In most cases, that is, dynamical systems that include positive feedback loops also include related negative feedback loops, which provide a check on the otherwise-unbounded amplification of the factors involved in the positive feedback loops. While positive feedback loops are self-reinforcing, negative feedback loops are self-limiting; in the same way that positive loops can lead to the rapid destabilization of dynamical systems in which they figure, negative loops can help keep dynamical systems in which they figure stable.

Consider, for instance, a version of the story of Romeo and Juliet in which the teenage lovers are somewhat more dysfunctional. In this version of the tale, Romeo and Juliet still respond to each others’ affections, but they do so in the opposite way as in the story told above. Romeo, in this story, likes to “play hard to get:” the more he sees that Juliet’s affections for him are growing, the less interested he is in her. Juliet, on the other hand, is responsive to 180