How Much Would You Be Willing to Pay to Reduce Murders by 30%?
I don't have a problem with assigning a dollar value to a life when discussing policy (there's generally no alternative), but I think this is the wrong way of framing the problem for a number of reasons. We are talking about getting a drug to the relatively small portion of the population that needs it by giving it to everybody. There are other options for getting lithium to the people who need it. The water supply approach has the advantage of missing fewer people though not all (I suspect this will make some people, particularly the paranoid, switch to bottled water), but it comes with other concerns.
The obvious comparison here is with fluoride, a comparison that Moffatt himself makes here:
Will it work? I don't know. It seems like it would be worthy a pilot study or two. Although those levels of elemental lithium are believed to be safe, there may be side-effects we are not considering. There are ethical considerations as well, but it is hard to make a case that adding fluoride to the water supply is ethical but lithium is not - and we've been adding fluoride to drinking water for over half a century.But there are at least two important differences between lithium and fluoride, and both differences have practical and ethical considerations.
First, tooth decay affects most people and virtually all children (whose health society has a responsibility to protect). There was no other practical way to get this treatment to everyone who needed it. Relatively few people need lithium treatment. As mentioned before, there may be other options for getting treatment to those people.
From an ethical standpoint, we are talking about exposing the majority of the population to a heightened level of a chemical that treats a condition that they don't have. This doesn't mean that adding lithium is a bad idea, but it is certainly possible to make an ethical case for fluoride that doesn't hold for lithium.
Add to that the concerns, noted by Joseph, over adding a mind-altering substance to a city's water supply. On the practical side, there have to be unexpected consequences (even with fluoride, there were enough minor side effects to reduce the level used). On the ethical side, you're adding a mind-altering substance to a city's water supply.
I don't know whether we should consider manipulating lithium levels, but I'm pretty sure we should start by acknowledging the complexity of the problem and taking a good look at the alternatives. Though much beloved by economists, this is one situation where "how much would you pay to..." is not going to cut it.