The benefits of pessimism may have arisen during our recent evolutionary history. We are animals of the Pleistocene, the epoch of the ice ages. Our emotional makeup has most recently been shaped by one hundred thousand years of climactic catastrophe: waves of cold and heat; drought and flood; plenty and sudden famine. Those of our ancestors who survived the Pleistocene may have done so because they had the capacity to worry incessantly about the future, to see sunny days as mere prelude to a harsh winter, to brood. We have inherited these ancestors’ brains and therefore their capacity to see
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