"One more page and I'll be sure"

At half past eleven you nearly stopped. You'd read enough to see that the odds were heavily in your favour. But heavily isn't entirely, and the thought came that the next page might settle it. So you opened it.

What you're after at that point is certainty, and it's worth asking whether any page could hold it. Doctors themselves don't deal in certainty about a twitching calf. They deal in likelihoods, in watching, and in ruling things out step by step if they need to. "Very rarely a sign of anything serious" is as close to certain as an honest medical writer can get in a sentence. If no page can promise you that you personally are fine, then the search has no finishing line, and you could read all night.

This matters more for some people than for others. Thomas Fergus surveyed 512 healthy adults in the United States about how often they searched for medical information, how anxious they were about their health, and how well they tolerated uncertainty. Intolerance of uncertainty means finding it hard to bear not knowing. For people who tolerated uncertainty well, searching had little connection to health anxiety. As intolerance rose, the connection grew stronger. The people who most needed the search to give them a definite answer were the ones for whom searching and anxiety went together.

A survey taken at one moment can't show which causes which. But it fits the experience of the search. If you go in needing to be sure, you come out more worried than you went in.

The search does end, though, so it's worth looking at how. You might expect someone in search of certainty to stop at the most convincing page. In my observation it stops at the frightening answer or at exhaustion, whichever comes first. I think that's because fear is the only thing in the search strong enough to interrupt it. The reassuring pages leave room for doubt, so you read on. The frightening page doesn't leave you wanting another page, because you're too scared by then to go on looking.

This is how a person can spend an hour finding out that a symptom is nearly always harmless and go to sleep convinced that they have the exception. They did read the evidence. The search happened to stop at the single page that pointed the other way.

The practical lesson is that the stopping point has to be decided before you start. If you wait to feel sure before you stop, you won't get there, and the search will stop at the frightening page. Chapter 15 suggests stopping when you've read what the page tells you to do.