President-elect Donald Trump named Betsy DeVos as his nominee for education secretary. Here's what you need to know about the conservative activist and billionaire donor. (The Washington Post)

A confirmation hearing for Betsy DeVos, President-elect Donald Trump’s nominee for education secretary, begins in the Senate on Tuesday. DeVos is known as an advocate for “school choice,” which means she has pushed for charter schools and voucher programs that use public funds to finance privately run schools.

Her support for school choice is controversial, not least because the efficacy of charter schools and vouchers is hotly debated. Supporters of school choice argue a couple of things: first, that it gives students who live in underperforming school districts a chance to go to “good” schools; and second, that by fostering competition between public and private schools, it pushes both toward excellence and efficiency. Critics insist that such programs drain funds from public schools.

Only a limited amount of U.S. data speaks to this debate. But a large-scale, countrywide experiment in school vouchers has taken place — in Chile. And Chile offers an instructive and cautionary tale about how school vouchers affect education.

How Chile’s school voucher system works

Chile has one of the world’s purest examples of a voucherized education system, established by the dictator Augusto Pinochet in 1980. The state guarantees a certain amount of money for each student’s education. These funds can be used in a public or private school.

Vouchers were introduced in Chile by free-market economists whose vision was similar to that of DeVos. When this vision held sway, from 1980-1990, public spending on education was cut essentially in half — from 5 percent to 2.5 percent of gross domestic product. The state did relatively little to regulate the private schools that took vouchers.

After Chile returned to democracy in 1990, a series of center-left governments increased spending on education. This gave public schools more funding. But some schools continue to struggle with operating costs — particularly schools in poor neighborhoods, schools that serve high-risk students, and schools with low enrollments. Since vouchers were introduced, public school enrollment has continued to decline. In 1981, 78 percent of students attended a public school. In 2013, 39 percent did.

How Chile’s system affected education

Lower-income students are less likely to attend a voucher school in Chile. In 2006, for example, 42 percent of students in the lowest income quintile attended a public school. Compare that with only 28 percent of students from the second-lowest quintile — and 4 percent from the wealthiest quintile of the population. Poorer students face barriers to using vouchers — including co-payments and difficulties with transportation. At least 63 of the country’s 345 municipalities do not have a voucher school option. These districts are mostly rural and poor.

After more than three decades, only mixed evidence supports the idea that Chile’s voucher system has improved student achievement. A few studies suggest that private schools that take voucher students have better test scores than public schools. Others find that this is mostly because private schools tend to admit more motivated and better-prepared students.

Until 2015, private voucher schools could limit which students could attend, thus cherry-picking the ones they thought would perform best. Unsurprisingly, then, any differences in test scores between public and private schools disappear after controlling for socioeconomic status.

The voucher system also widens the achievement gap between the haves and the have-nots. The aggregate socioeconomic status of a school more strongly predicts test-score achievement in voucher schools than it does in public schools. This suggests that voucher schools fail to level the playing field. Instead, the voucher system has aggravated Chile’s high levels of inequality.

Chile’s backlash against vouchers

The voucher system has recently sparked a public backlash. Between 2006-2011, high school and university students took to the street to protest the poor quality of public education. But efforts to improve public education and regulate voucher schools have often been stymied by proponents.

A 1995 change that mandated an extended school day for all children — thereby ensuring that public and voucher school students had similar in-class time as children attending tuition-paying private schools — met opposition from voucher schools because of the associated costs. To overcome the resistance, politicians agreed to use public funds to finance improvements to voucher school infrastructure. In some cases, that meant delaying similar improvements in public schools.

Meanwhile, private schools and right-wing parties blocked a proposed 2009 change that would have struck down any admissions criteria for schools using public funds — including private schools that took vouchers. Only in 2015, after waves of intense protest, was President Michelle Bachelet able to pass a bill that outlawed admissions criteria for voucher schools and increased public regulation and oversight of schools using public funds.

What about in the United States?

Of course, Chile differs from the United States in many ways. But 14 U.S. states have large-scale voucher programs and other policies that use public funds to send students to private schools. And the evidence from these systems is similar.

As in Chile, U.S. school choice programs can end up reinforcing class and race inequalities unless closely monitored. When schools are monitored, school choice has increased racial integration in schools in highly segregated areas.

President-elect Donald Trump listens as Betsy DeVos speaks at a “USA Thank You Tour 2016″ event at the DeltaPlex in Grand Rapids, Mich. on Friday, Dec. 9, 2016. DeVos’s confirmation hearings begin Tuesday. (Photo by Jabin Botsford/The Washington Post)

But in two cities with prominent school choice programs — Milwaukee and Cleveland — these programs did not always serve the neediest students. The growth of vouchers coincided with the growth of white students using public funds. And in one study of Cleveland’s program, the students who did not use vouchers were poorer and more likely to be nonwhite than students who did use them. This resembles Chile’s pattern.

U.S. voucher systems can also exclude students with special needs. Federal law requires public schools to serve students with disabilities, but some states with choice programs allow participating private schools to reject students with disabilities. Even when serving a student body that is racially and socioeconomically comparable to the public school population, charter schools enroll a lower share of special-needs students than the surrounding public schools.

How does school choice affect student achievement? Much of what we know is based on studies of Milwaukee, Cleveland and Florida, which have the longest-running programs in the United States. In general, as in Chile, evidence is mixed. Conclusions depend strongly on researchers’ methodological choices and the think tank or research organization with which they are affiliated.

For example, one study of Florida schools concluded that failing schools improved when vouchers let students go elsewhere. But another researcher who examined the same schools but used different methods could not replicate the results.

Peer-reviewed studies are considered the most rigorous evaluations of vouchers, but these are few. Although some studies found that students using vouchers had higher test scores than their public school peers, in general, peer-reviewed studies found that the performance of voucher students does not differ significantly from that of public school students.

A major shortcoming of voucher evaluations is that they cannot identify the reason that vouchers work. For instance, a study of Dayton, Ohio, schools found that vouchers had a small positive impact on some — but not all — students’ academic achievement. For instance, African American students’ test scores went up, but the scores of whites and Latinos did not. Another study of Milwaukee found that vouchers were associated with higher scores in math but had no impact on reading scores.

Why would the same voucher program be effective for some, but not all, racial groups or academic subjects? Without understanding how vouchers work, it is difficult to say whether a program that works in one place would work in a different setting or on a larger scale.

Overall, peer-reviewed research in the United States has found that vouchers have had little, if any, effect on student academic performance or education quality in public schools competing with private voucher schools. Only a few places, however, have used vouchers, making Chile’s fully voucherized school system a cautionary tale for American reformers.

In Chile, the voucher system has not improved education opportunities for many poor or rural children. Rather, it has increased socioeconomic inequalities and provoked discontent and protest.

Jennifer L. Erkulwater is associate professor of political science at the University of Richmond, and has written extensively on U.S. social welfare politics and policy.

Jennifer Pribble is associate professor of political science and international studies at the University of Richmond, and author of Welfare and Party Politics in Latin America.