Some topics evolve by occasional big news events interspersed by long periods of little or no news. The public reacts dramatically to the big news events and seems to ignore the slow news.

For example, a terrorist attack is followed by general paranoia and a tightening of security. But no matter how much time passes without another attack, there never seems to be a restoration of the old equilibrium. News is like a ratchet with each big reaction building directly upon the last, and the periods in-between only setting the stage for the next.

The usual way to interpret this is an over-reaction to the salient information brought by big news events, and a failure to respond to the subtle information conveyed by a lack of big news. We notice when the dog barks but we don’t notice when it doesn’t.

But even a perfectly rational and sophisticated public exhibits a news ratchet. That’s because there is a difference between big news and small news in the way it galvanizes the public. Large changes in policy require a coordinated movement by a correspondingly large enough segment of the population motivated to make the change. Individuals are so motivated only if they know that they are part of a large enough group. Big events create that knowledge.

During the slow news periods all of these individuals are learning that those measures are less and less necessary. But that learning takes place privately and in silence. Never will enough time pass that everyone can confidently conclude that everyone else has confidently concluded that …. that everyone has figured this out. So there will never be the same momentum to undo the initial reaction as there was to inflame it.