According to Skinner, we, too, in most aspects of our lives, are like pigeons pecking at a button to receive little snacks. And this, according to the cognitive scientist Tom Stafford, explains the check-in impulse behind email and other online technologies. Unlike a food pellet, email isn’t always rewarding; in fact, it is often annoying (though with fewer people and less spam, it was surely more rewarding back in the 1970s). Once upon a time, there could be no new email for days at a time (few of us have that problem now). Much of what we get is uninteresting or indeed difficult to deal with. But every so often we get a message we are very glad to have. That such “rewarding” email comes unpredictably does not dim its allure or keep us from looking for it. On the contrary: as Stafford points out, the most effective way of maintaining a behavior is not with a consistent, predictable reward, but rather with what is termed “variable reinforcement”—that is, rewards that vary in their frequency or magnitude.

Tim Wu, The Attention Merchants, loc. 653