In short, at least some — and perhaps all — of the cost of universal tuition-free public higher education could be defrayed by redeploying money that the government is already spending.

Some form of tuition-free college already exists or soon will be in place in 13 states, and some of this cost would not need to be duplicated to achieve universal tuition-free public college.

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The existing tuition-free state programs apply almost exclusively to two-year colleges (New York’s Excelsior scholarship is an exception), and they are funded by increasingly tight state budgets, which threaten their long-term viability.

The federal government has much deeper pockets than the states do, but it mostly reduces tuition costs indirectly by providing financial aid to low-income students. The largest program is the Pell Grant, which provides a maximum award of about $6,000 per year.

Thanks to various forms of financial aid, for the neediest students, some public colleges are already tuition-free. So why not simply expand financial aid to provide larger dollar amounts to more students? The problem is that need-based aid requires a determination of eligibility, and students usually don’t know what they will pay for college until after they decide to apply.

Pell Grant aid helps low-income students attend and graduate from college, but it has little impact on the decision to enroll in the first place. Financial aid is helpful but only for low-income students who can successfully navigate the system.

The simple promise of zero tuition, whatever your family income, is in itself an effective nudge that is likely to attract more low-income students to college. That is unequivocally a good thing. But such a promise will also attract some wealthier students, who would go to college anyway, so reduced tuition would be just a windfall for them.