cases a seed infects too few people and the disease or idea dies out. Peter Dodds and Duncan Watts (of the Watts-Strogatz network paper) of Columbia University have shown that what happens can depend on how much more likely a second exposure is to infect an individual than a first exposure. Their analysis suggests that the spread of diseases or ideas depends less on “superspreaders” or opinion leaders than on how susceptible people are—how resistant they are to disease or how adamantly they hold their current opinion. Such results imply that the best way to hamper or advance contagion would be strategies that increase or reduce the odds of infection. Better health procedures, for instance, or financial incentives to change voting preferences, could tip the future one way or another.

“Our results suggest that relatively minor manipulations … can have a dramatic impact on the ability of a small initial seed to trigger a global contagion event,” Dodds and Watts declared in their paper.8 It sounds like just the sort of thing that Hari Seldon incorporated into psychohistory, so that his followers could subtly alter the course of future political events.

In real life, of course, people don’t necessarily transmit opinions or viruses in the simple ways that such analyses assume. So some experts question how useful the statistical mechanics approach to society will ultimately be. “I think in some limited domains it might be pretty powerful,” says Cornell’s Steven Strogatz. “It really is the right language for discussing enormous systems of whatever it is, whether it’s people or neurons or spins in a magnet.… But I worry that a lot of these physicist-style models of social dynamics are based on a real dopey view of human psychology.”9

Of course, that is precisely where game theory comes into play. Game theory has given economists and other social scientists the tool for quantifying human psychology in ways that Freud could only dream of. Neuroeconomics and behavioral game theory have already sculpted a much more realistic model of human psychology than the naive Homo economicus that lived only to maximize money. And once you have a better picture of human psychology—in particular, a picture that depicts the psychological



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