We don't know a lot-- this is true for all of medicine, not just psychiatry, but it's true for psychiatry too. Sometimes we think we do, and we proclaim truths that might just not be.
So I'm going to refer you to a number of sites to see what you think.
Over on KevinMD, Edwin Leap writes about "When self-evident truth in medicine is systematically ignored." Dr. Leap writes:
And yet, medicine is filled with situations in which “self-evident truth” is systematically ignored, and those who believe in it intentionally and often viciously marginalized.
For example, after years of being told that physicians weren’t giving enough treatment for pain, and after years of clinicians saying, “yes we are, and too many people are addicted and abusing the system,” the data from CDC says that far too many are dying from prescription narcotics, far too many infants being born addicted, and far too many people, young and old, are using analgesics and other drugs not prescribed for them. To which many of us say, “duh!”
Today's New York Times "most emailed list" includes articles which question our assumptions or assert new truths :
1) Gary Taubes writes in "Salt, We Misjudged You," :
I add as an addendum that I think I missed the lecture in medical school where salt was the "Public Enemy No. 1." I somehow got through physiology and pathology believing that if a person has normal kidney function and doesn't suffer from congestive heart failure or have problems with fluid retention, that excess salt gets excreted and isn't associated with shortening your life. (The issue of CHF was a big one though, for those who had it, a bowl of soup could land them in the ICU). So Gary Taubes is the same guy who wrote a NYTimes Magazine article 10 years ago that gave credibility to the Atkins diet and challenged our beliefs that pasta and carbohydrates and a low-fat diet are good for you in What If It's All Been a Big Fat Lie?
I think we all have beliefs about what it's healthy to eat, but we don't have real answers and I imagine that the answer is more related to individual biology then anything we've come to entertain as a society with our food pyramids. We'll have to see if New Yorkers live longer when they can't get super-sized soft drinks.
2) Jane Brody writes in Slathering on Sunscreen, Early and Often, about the necessity of teaching children to put on sunscreen like we teach them to fasten their seat belts. She gives statistics for early sun exposure and the increased risk of skin cancers. It's all said as a given, and maybe she's right, but commenters wrote in asserting that sunscreens cause skin cancer, that the chemicals in them are toxic, that the evidence is lacking, that sunscreen blocks the light rays needed for Vitamin D production and all the problems associated with low Vitamin D levels, and there were examples of populations that spend more time in the sun but have lower rates of skin cancer. No answers from me, but I'm not as certain as Jane Brody that the evidence is there to suggest that sunscreening everyone will lead to longer, happier, or healthier lives, and I'm not sure it won't either. I, for one, don't like the smell.
3) Exercise is good for you, right? I actually do believe that. But Gina Kolata writes in For Some, Exercise May Increase Heart Risk that 10% of people got worse, after exercise, on at least one measure (meaning a lab value or their blood pressure) related to heart disease.
The problem with studies of exercise and health, researchers point out, is that while they often measure things like blood pressure or insulin levels, they do not follow people long enough to see if improvements translate into fewer heart attacks or longer lives. Instead, researchers infer that such changes lead to better outcomes — something that may or may not be true.
4) Finally, in The Trouble With Doctor Knows Best, Peter Kahn writes: