Automatic enrollment in retirement accounts counts as the most successful nudge yet tested on a large scale. It also underscores the limitations of the strategy.

America’s shortcomings are serious. The United States has the deepest poverty rates among rich countries. It is at the bottom of the pile in terms of infant mortality, obesity and diabetes, and has the highest teenage pregnancy rate among rich members of the Organization for Economic Cooperation and Development, the club of advanced industrial nations. The death rate from drug overdoses among young white adults is now about as high as the death rate from AIDS was at its peak in the mid-1990s.

It’s great to know that there are promising ways to improve society by developing a smarter email or changing the default choice on an application form. But if the question is whether policy makers can cheaply nudge Americans out of destitution onto a path to prosperity, the answer must be no.

The limits of nudging come into focus in an evaluation by the research firm MDRC of a set of behavioral nudges intended to increase poor fathers’ payment of child support for their children.

In Franklin County, Ohio, careful reminders increased the share of noncustodial parents who paid support, to 51.5 percent from 48.5 percent. In Cuyahoga County, Ohio, text message reminders increased the payment rate to 49.8 percent from 47.3 percent. In both cases, the researchers observed, the payments were quite small.

If the government’s objective was to change the landscape of child support — a critical policy question considering there are 25 million children living with a single parent — this will not be enough.

Policy experimentation is always welcome. But the approach taken in this instance seems to spring from a shaky assumption: Poor noncustodial fathers can afford child support but choose not to pay it. As the MDRC researchers put it, “the modest findings suggest that the targeted noncustodial parents may have a limited ability to pay.” Dealing with that, however, might cost taxpayers real money.