{mosimage}It’s hard not to play along when your children come up with cute untruths.

For example, for a time one of my kids used to tell me confidently and matter-of-factly that if he didn’t go to bed, the moon would not rise. It was a reasonable conclusion based on his personal experience at the time (he was about two or three). Later, of course, his bedtime changed, he began to notice the moon in the sky even during some days, and concluded differently.

Unfortunately, many politicians have yet to grow out of the toddler phase when it comes to spotting and discarding spurious correlations. They insist, confidently and matter-of-factly, that without their favorite spending program or regulation, some huge chunk of the economy would cease to be or that ever-improving health and safety trends would suddenly reverse themselves.

The most naĢve and destructive examples of such thinking stem from misusing the concept of the multiplier effect. You’ve seen or read this many times, I’m sure. A politician will say that for every dollar spent on such-and-such a project, the public will receive multiple dollars back in economic activity and thousands or even millions of jobs. In virtually every case, the statement isn’t just invalid. It’s idiotic. And yet it just gets restated by the next earnest-sounding politician.

Most of the time, such politicians are citing an economic-impact study that takes the amount spent and runs it through a model that estimates the local expenditure on labor and materials and the resulting employment implications. While such data can be useful ą particularly if you are thinking about going into the business of supplying the labor or materials in question ą they don’t speak at all to the net economic benefits.

Getting to the net requires that you estimate the benefit of using those dollars on some alternative. Economists call this the opportunity cost. Basically, all costs are opportunity costs, whether they are denominated by dollars, time, or some other means. If you spend $8.50 eating lunch at Jersey Mike’s (highly recommended, by the way) you can’t spend the same $8.50 on some other meal, or on buying socks at the store after having skipped lunch. More broadly, the resources you consumed getting to and from the sub shop, including the minutes, can’t be devoted to something else. What you didn’t consume ą the alternative meal, the socks, the extra time at the office ą constitute the opportunity cost.

In public finance, the opportunity cost comes at two stages. Certainly, the tax dollars you spend on, say, highway construction can’t be spent on public schools or law enforcement. But there is also an opportunity cost to converting private dollars, earned through voluntary means, into tax dollars. When people keep more of what they earn, that money doesn’t disappear just because it no longer shows up in the government’s balance sheet. It is devoted either to current private consumption or to net private investment, both of which have economic impacts, too. 

The only real justification for a government program is that private individuals, spending a given amount of money through voluntary exchange, won’t get as high a return on that money as the government would by taxing the money from them and devoting it to a public purpose.

The case isn’t that hard to make when it comes to basic governmental services such as law enforcement and the courts. Beyond that, you have to argue that policy makers are likely to know better than citizens how best to spend the citizens’ own money. There are such cases, I would submit. But these cases are rare.

Those who assert the magic of multiplier effects to justify their pet programs may be dissembling. But it is my experience that most of the time, they don’t know enough about the matter to be lying. They are just repeating what they’ve heard, or spotting spurious connections on the basis of limited experience.

It’s their business if they choose, Peter Pan-like, not to grow up. But they should keep their hands out of the wallets of the grownups.