Think for a minute about activities that entrance their practitioners, like gambling, shopping, or fishing. They all, in fact, have variable and unknowable reward schedules. A slot machine that rewarded every pull or even every third would offer no thrill; and no one hunts cows for sport. But take away the certainty and the real fun begins. Likewise, Stafford argues, “checking email is a behavior that has variable interval reinforcement….Everyone loves to get an email from a friend, or some good news, or even an amusing web link.” It is enough to have had such an experience a few times to get you regularly fishing for it; constant checking is thus reinforced, “even if most of the time checking your email turns out to have been pointless. You still check because you never know when the reward will come.”

Tim Wu, The Attention Merchants, loc. 655