Mitt Romney wants to give families a child allowance and conservatives are all atwitter. Here at TAC, Patrick T. Brown calls the Romney plan “an exciting first step towards an authentically pro-family politics.” But wait: Over at the New York Times, Oren Cass delves into the details and dissents, arguing that “monthly cash payments should go only to working households” rather than be distributed universally regardless of job status.

Normally when pundits chatter about a “conversation worth having” or an “idea whose time has come,” the exact opposite is true. But in this case, it really is good that we’re having a discussion about family policy. Along with too many of our societal institutions, the family has been crumbling as of late, despite its being the most essential building block of our civilization. Families are our foremost way of underwriting our national future, that all-important first concentric circle between the individual and atomized oblivion. We ought to bolster them, and there’s nothing wrong with asking whether a federal program might be able to help.

But let’s also bear in mind that government alone can’t rebuild the family, that we need to make sure we keep our expectations in check.

What government could do, and what Romney’s plan might do, is reduce child poverty. This is the sort of simple linear outcome that the state can handle: If you give people more money, they’ll be less poor. Research from the European Union, where many countries have had child allowances in place for decades, tends to bear this out, though even there we need to be careful. When Ezra Klein declares that “estimates suggest the Romney proposal would cut child poverty by a third,” there isn’t an NFL training camp in America that’s doing as much heavy-lifting as that word “suggest.”

Some conservatives have lately sounded similarly grandiose, touting a child benefit as not just a boon to existing families but a way to boost America’s birthrate, which plummeted during the 1960s, stagnated throughout the ’80s and ’90s, and sagged even further after the Great Recession. These natalists think the Romney plan could provide an incentive for mothers and fathers to have more kids by helping to offset the enormous financial commitments of parenthood. Yet this may be hoping for too much. To examine why, let’s again turn across the pond to Europe.

Start with the Scandinavian countries. They not only have generous child allowances, but lengthy maternity and paternity leave and other pro-family social welfare measures in place. Yet their birthrates are not significantly higher than America’s (Sweden’s is a tick better, Finland’s is lower). The European nation with the highest birthrate is, believe it or not, lily-livered libertine France, yet even French fertility has never come anywhere close to where it was 60 years ago, despite that country having its own child allowance system.

European birthrates have gone up on occasion: Sweden saw a stark baby boom beginning in the mid-1980s that went bust a decade later. But the overall birthrate picture throughout the big-spending E.U. is one of decline and stagnation, with pro-family policies having done little to arrest that trend. It’s true that the United States is notably not Europe; it’s also true that we’ve never aggressively secularized in the same way that Sweden and France have. It’s further true that Hungary is trying hard to prove the European exception, as Viktor Orbán aggressively pushes pro-family policies in the name of natalism.

But none of that is enough to counter the greater thrust of the data. How much would the Romney plan boost the American birthrate? Judging by the comparative evidence, the most likely answer is: not at all.

And in a way, we should be grateful for that. One of the great pleasures in this life is watching overly ambitious policy wonks be proven wrong time and again. Education overhauls like No Child Left Behind largely failed to improve test scores; Obamacare didn’t contain rising costs; Cash for Clunkers should still be a national punchline. Man isn’t a guinea pig who can be directed by tinkerers in a federal laboratory; it would be a dull and depressing world if he was. When the objects of policy become too complex, the central planners quickly discover they have a knowledge problem, that they can’t hope to manipulate human activity according to their designs. Instead they end up with unplanned costs, unintended consequences, and graph lines that stubbornly refuse to budge.

So it goes with the family, whose decline is far too multifaceted to be plotted on a wage-growth flowchart. Among its causes: the availability of birth control, which allows for more family planning; a cultural emphasis on having a career, which leads moms and dads to postpone children in favor of professional accomplishment; the simple and obstinate fact that a lot of women want to work outside the home; a campaign that begins in high school guidance counselor’s offices to push students into the best and priciest colleges with childrearing scarcely a consideration; apps like Tinder that are little more than fire-sale flesh bazaars, which discourage men in particular from taking dating seriously and settling down; a barbaric student loan system; and much more.

That doesn’t mean we shouldn’t consider the Romney legislation. But it does mean accepting its limitations and acknowledging that a lot of the change being sought can’t come from the top down. Federal policy is what it is: a blunt instrument, not magic, not a shortcut.