The prominence of school vouchers continues to surge across the country — but they might not benefit the families who need them the most.

Over the past few years, states like Ohio and Arkansas have expanded their school voucher programs to allow most or all parents to receive funding to send their kids to private schools. More than 20 states now have some kind of voucher program with more in consideration. Arizona was the first state to create a universal voucher program in 2022 — and experts have said it’s the state to watch when analyzing the impact of vouchers for all.

The modern school voucher movement started to grow in the 1990s under the idea that the government would give parents a certain amount of money to put toward private school tuition. The programs were means-tested, meaning recipients had to meet a certain poverty limit to receive assistance, with the idea that kids with fewer resources would be able to earn a better education at private schools.

However, gradually, more states began to raise the poverty limit, making nearly any parent eligible to receive the funding — and in some states, it led to the cash going to the wealthiest families. Arizona is “a cautionary tale” regarding the expansion of vouchers, Josh Cowen, professor of education policy at Michigan State University, told Business Insider.