Homo sapiens emerged as a species about 200,000 years ago and soon—with fire, simple tools, and an egalitarian social system—we inhabited every continent. But now our infinitely elaborated tools and our highly non-egalitarian social system threaten sustainability. Many at the bottom lose the desire to live, committing suicide or succumbing to drugs, alcohol, and obesity. As women gain education and contraception, they lose the desire to reproduce, and our species' total fertility plummets toward levels far below replacement.

What is Health? explains from the bottom up—molecules to society—how we reached this point. The individual organism was designed by natural selection to maximize performance for a given energy cost. Our brain predicts what will be needed and controls metabolism, physiology, and behavior to deliver just enough, just in time (allostasis). By preventing errors, rather than correcting them, energy is saved. Predictive regulation requires learning governed by an optimal rule that rewards each positive surprise with a pulse of dopamine (experienced as a pulse of satisfaction) that encourages the behavior.

But now obtaining food and comfort without surprise, we are deprived of the small rewards upon which rest the whole edifice of behavioral regulation and mood. Lacking frequent pulses, we grow restless and seek new sources via consumption: more food and more drugs that produce great surges of dopamine. But the next surprise must be more—to which our systems adapt by reducing their sensitivities—which drives us into spiraling addictions.

Standard medicine treats addictions by blocking reward circuits. But this strategy prevents satisfaction and deepens despair. Standard economics promotes “growth” for more “jobs”. But “jobs” devoid of challenge and social rewards drive despair. To restore health, we must re-expand opportunities for small satisfactions and thereby rescue the reward system from its pathological regime.

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