Do we know what we don't know

A few items over the course of the past few months gives pause to what we think we know or don't know.

1. Fluoride, we know has beneficial effects - but how much fluoride is better? This news item reverses what we thought we knew:

The U.S. Department of Health and Human Services announced plans Friday to lower the recommended level of fluoride in drinking water for the first time in nearly 50 years, based on a fresh review of the science.

The announcement is likely to renew the battle over fluoridation, even though the addition of fluoride to drinking water is considered one of the greatest public health successes of the 20th century. The U.S. prevalence of decay in at least one tooth among teens has declined from about 90 percent to 60 percent.

The government first began urging municipal water systems to add fluoride in the early 1950s. Since then, it has been put in toothpaste and mouthwash. It is also in a lot of bottled water and in soda. Some kids even take fluoride supplements. Now, young children may be getting too much. ...

One reason behind the change: About 2 out of 5 adolescents have tooth streaking or spottiness because of too much fluoride, a government study found recently. In extreme cases, teeth can be pitted by the mineral — though many cases are so mild only dentists notice it. The problem is generally considered cosmetic and not a reason for serious concern.

The splotchy tooth condition, fluorosis, is unexpectedly common in youngsters ages 12 through 15 and appears to have grown more common since the 1980s, according to the Centers for Disease Control and Prevention.

But there are also growing worries about more serious dangers from fluoride.

The Environmental Protection Agency released two new reviews of research on fluoride Friday. One of the studies found that prolonged, high intake of fluoride can increase the risk of brittle bones, fractures and crippling bone abnormalities.

Critics of fluoridated water seized on the proposed change Friday to renew their attacks on it — a battle that dates back to at least the Cold War 1950s, when it was denounced by some as a step toward Communism. Many activists nowadays don't think fluoride is essential, and they praised the government's new steps.

See also this SciAm article (subscribers only).

2. This SciAm (subscribers only) article on vitamin D was so convincing that not long after, I went out and got myself some vitamin D supplements. It is unusually convincing (to me) and it is hard to place a reason for this - especially since there are no randomized trials involved. Theoretical links between vitamin D and health were explored and found to be true. There were no size effects and least of all, very little on recommended dosage. Perhaps it was because both the authors were so convinced that they are now on vitamin D supplements as well (each of them on a different dose!).

Not long after reading that, Freakonomics reports on two apparently conflicting headlines on vitamin D by the NYT and WSJ. However, the headlines in these cases actually tell the same story:

A long-awaited report from the Institute of Medicine to be released Tuesday triples the recommended amount of vitamin D most Americans should take every day to 600 international units from 200 IUs set in 1997.

Given that the so-called 'hard' sciences are struggling with the dosage issue, it was refreshing that economists were able to say that they know what they don't know. Here is Mankiw on extending unemployment benefits:

So when I hear economists advocate the extension of UI to 99 weeks, I am tempted to ask, would you also favor a further extension to 199 weeks, or 299 weeks, or 1099 weeks? If 99 weeks is better than 26 weeks, but 199 is too much, how do you know?

It is plausible to me that UI benefits should last longer when the economy is weak. The need for increased aggregate demand is greater, and the impact on job search may be weaker. But this conclusion is hardly enough to tell us whether 99 weeks is too much, too little, or about right. It is also conceivable that the amount of UI offered in normal times is higher than optimal and that a further extension would move us farther from what is desirable.

Likewise, given what economists know, can policies that target inflation at 3 percent is better than one that targets inflation at 4 percent?