"Outdated medical lore"

I loved this essay from Newsweek that explains why we STILL think we need to drink 8 glasses of water a day. Here's an excerpt:

Now, the authors are back with Don't Swallow Your Gum! (Griffin Original), a book of medical myths and half-truths that will be published next week. Among the 66 myths, there's something to surprise everyone: that, despite what Mom told us, vitamin C does not cure a cold and even the highest SPF sunscreen will not prevent all sunburns. But what's more surprising than the myths they debunk, is how strongly their friends, colleagues and readers protested their research. Both Vreeman and Carroll have been repeatedly told they're incorrect, misinformed or flat-out wrong, that these are medical facts they're messing with. "It's not like we discovered something new, we just reviewed the literature," says Carroll. "But people still won't take it, it's like nothing would be enough to convince them otherwise."

Why do we believe and cling to these seemingly unimportant nuggets of information? Why is it so difficult for us to accept that reading in poor light won't ruin our eyesight and we don't actually need to drink eight glasses of water each day? Turns out, we're pretty likely to side with the things our doctors and parents have told us, myths we've seen reinforced with our own eyes, even when research tells us to believe otherwise. "People often take rumors or anecdotes as fact," says Vreeman. "We tend to give those things as much weight as we would a scientific study because they're connected to people in charge. We sometimes reason things out after the fact, come up with patterns to explain what we saw happen."

The body of research on belief formation is relatively sparse. One expert in the field, York University psychologist James Alcock, admits that it's difficult to trace where beliefs start. "Even as individuals we usually can't explain where beliefs come from," says Alcock, who is currently at work on a book about the psychology of belief. "Why should you drink eight glasses of water? People will say they heard it somewhere. Sometimes it's impossible to trace the source, but it just gets repeated over and over." Some myths begin with a kernel of truth that gets misinterpreted, like the eight-glass theory. In 1945, the Food and Nutrition Board of the National Research Council stated that adults should take in about 2.5 liters of water a day and that most of this is contained in prepared foods. Ignore that last part of the recommendation and you've got the eight-glass mandate. Others are couched in what seems like common sense, like the idea that reading in the dark is bad for the eyes. After all, our eyes do hurt after reading in the dark, so it makes sense to assume that some kind of damage is taking place.