MORE YOUNG people would increase their earnings if they were able to complete a four-year college degree, and American society as a whole would benefit from this enhancement of human capital. But while 69 percent of high school graduates at least start postsecondary education, only about half of them eventually get a B.A. or equivalent. This gap between aspiration and performance is partly due to the price of higher education; a host of proposals from Democratic presidential candidates to make college free, or at least “debt-free,” seeks to address the situation. The problem, as always, is the cost, to government, of these plans, and the possibility that programs will spend billions bolstering “affordability” not only for the needy but for many upper-middle and upper-income families who could get by without taxpayer help.

What’s the most cost-effective approach, according to the latest research? A study presented recently at a Brookings Institution annual conference suggests that one popular idea — free community college — would actually lead to slightly lower college completion, in part because financially challenged students are already well-served by federal aid such as Pell Grants, and because the offer of free two-year degrees might draw students away from four-year programs. “The predominant effect of the policy,” the study notes, “is to induce students who would not otherwise enroll to complete some time in college without attaining a degree.”

The study also hints at a possible unintended negative consequence of slashing tuition at public four-year institutions: It would undercut their private-college competitors, which currently enjoy the highest graduation rates, but which would lose enrollment to public institutions if the latter’s tuition were cut by just 10 percent, let alone reduced to zero. Interestingly, a policy that combined free community college with a 30 percent to 40 percent reduction in four-year public college tuition would result in no change in current college-completion rates, the study found.

Ultimately, the most cost-effective method for boosting them is simple and straightforward, according to the study: increasing spending on instructional programs at public colleges, which, unfortunately, is precisely the opposite of what has been happening at many state schools since the 2008-2009 economic recession. A close second, in terms of increased graduations per dollar of spending, is a policy that reduces tuition and fees but targets the reductions to low-income students who need them most.

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