On “Fox News Sunday,” Chris Wallace pressed Rick Santorum on his plan to triple the federal tax credit for children, in the hopes that Americans will have more babies. The host told the presidential candidate that the Wall Street Journal editorial page complained that Santorum is “essentially agreeing with liberals who think that the tax code should be used to pursue social and political goals.”

I rather enjoyed the former senator’s response.

“What’s happening in this country — we’ve seen a dramatic increase. The child adoption credit or deduction back then used to be 10 times, almost 10 times what it is today. When the government had a policy that said, we want human capital, we need and actually want children to be here in America and the government has a policy of helping and supporting families because children are the greatest resource. They’re the natural resource that creates wealth in this country.

“And if it wasn’t for immigration, our population would be declining. And one of the biggest reasons, Chris, is the financial burden on families. And the federal government over the years has year by year by year decreased support for families. And guess what’s happening? Year by year by year, birth rates are going down.

“This is not social engineering.”

No, of course not. This is simply a plan in which federal officials would decide to use the power of government taxation to encourage Americans to engage in a certain behavior, in order to advance a larger societal agenda.

Who said anything about “social engineering”?

Look, we could have a conversation about the impact of economic instability and birth rates. But whether Santorum’s plan constitutes social engineering isn’t really in doubt.

Republicans, most notably social conservatives, are far fonder of the practice than they like to admit. Kansas Gov. Sam Brownback (R), for example, sought federal grants to pay for counseling that encourages unwed parents to marry and free marriage licenses for those who do.

In the abstract, the right is supposed to be offended, at a fundamental level, by the idea of using federal resources to alter how people can and will behave. It’s supposed to be anathema for anyone who values “limited” government.

But in practice, when it comes to child tax credits, abstinence programs, marriage counseling, etc., conservatives think social engineering is a great idea. It’s foolish for Santorum to argue otherwise.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.