rather more colloquial than what you usually find in a neuroscience journal). “Do I chase this new prey or do I continue nibbling on my last kill?” Berns and Montague wrote in Neuron. “Do I run from the possible predator that I see in the bushes or the one that I hear? Do I chase that potential mate or do I wait around for something better?”2

Presumably, animals don’t deliberate such decisions consciously, at least not for very long. Hesitation is bad for their health. And even if animals could think complexly and had time to do so, there’s no obvious way for them to compare all their needs for food, safety, and sex. Yet somehow animal brains add up all the factors and compute a course of action that enhances the odds of survival. And humans differ little from other animals in that regard. Brains have evolved a way to compare and choose among behaviors, apparently using some “common currency” for valuing one choice over others. In other words, not only do people have money on the brain, they have the neural equivalent of money operating within the brain. Just as money replaced the barter system— providing a common currency for comparing various goods and services—nerve cell circuitry evolved to translate diverse behavioral choices into the common currency of brain chemistry.

When you think about it, it makes a lot of sense. But neuroscientists began to figure it all out only when they joined forces with economists inspired by game theory. Game theory, after all, was the key to quantifying the fuzzy notion of economic utility. Von Neumann and Morgenstern showed how utility could be rigorously defined and derived logically from simple axioms, but still thought of utility in terms of money. Economists continued to consider people to be “rational” actors who would make behavioral choices that maximized their money or the monetary value of their purchases.

Putting game theory into experimental action, though, showed that people don’t always do that. Money—gasp—turned out not to be everything, after all. And people turned out not to be utterly rational, but pretty darn emotional. Imagine that.

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