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Smoke detector

If the body is so well designed, why is life so full of apparently unnecessary suffering? Anxiety, pain, cough and fever are all normal and useful, but it seems obvious that the regulation mechanisms are set to make life far worse than it needs to be. For evidence, sonsider that we can and do use drugs to safely block much suffering.  After mulling this over for several years, I realized that natural selection should shape these regulation mechanisms according to the principles of signal detection theory.  This is the same theory that electrical engineers use to optimize signal to noise ratios.  In the case of evolved defenses, the system must respond when there is a real threat, but not otherwise. However, most cues leave it uncertain if the danger is actually present.  In such cases, the threshold at which a response is optimal depends on the prevalence of signals to noise, and the costs of a false alarm and the costs of not expressing the defense when the danger is actually present.  I could not at first believe my calculations. They showed that when defenses are inexpensive, as they often are, false alarms will vastly outnumber responses to real danger. This is normal and optimal, albeit terribly distressing.  The smoke detector principle provides a crucial guide for all clinical thinking about when it is safe to block defensive responses. 

Nesse, RM: Natural selection and the regulation of defensive responses. Evolution and Human Behavior, 26(1) 88-105, 2005.


See the chapter in Why We Get Sick on defenses for the first clear statement of the Smoke Detector Principle. 
Of course, it is really just a restatement of Pascal's Wager.  
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