When Donald Trump set out to pick the next education secretary, he faced a stark choice. He could choose an insider who had shaped education policy for a state or large school district. Or he could bring in an outsider — someone who views traditional public schools as a failed system in need of dismantling.

He picked an outsider.

Unlike most of her predecessors, Betsy DeVos, Trump’s nominee for education secretary, has never taught in a public school or college, run a school district or public university, served on a school board, or shaped state education policy. DeVos, a billionaire philanthropist, instead made her name as an advocate for school vouchers — the idea of letting students use public money to attend private schools.

Together, DeVos and Trump want to oversee the biggest change to American public education in half a century. Trump’s plan for his first 100 days includes a $20 billion federal voucher program for children living in poverty, a program he’d likely pay for by dismantling the biggest existing system of federal support for public schools.

For some conservatives and Republicans in Congress, the plan would be a dream come true. Voucher supporters believe that competition from private schools will provide a better education for all children, and that offering poor families the opportunity to choose a school, including a private school, is a matter of social justice — since well-off families already can exercise choice by buying a house in a good school district.

But although some research has shown a small benefit for voucher students or the public schools they left behind, many other studies have shown no effect. One thing, though, is certain about Trump’s plan: It would send a generous amount of taxpayer money to private schools, including religious schools, and public schools would lose some of their state and federal funding while serving millions of disadvantaged students who would remain in the system.

How Trump’s school voucher plan would work

The ultimate goal of Trump’s plan is for states and the federal government together to offer a school voucher to every child living in poverty in America — exploding the number of students getting vouchers from 170,000 to 11 million, and ending a program that provides federal money to more than half of all American public schools.

Trump says he’d pay for the $20 billion program by cutting elsewhere in the education budget. That almost certainly includes the $14 billion the federal government spent in 2015 on grants to educate disadvantaged children, defined as children from families making at most 185 percent of the poverty line (just under $45,000 for a family of four). The grants, known as Title I grants, go to states and districts to pass on to schools based on the proportion of disadvantaged students those schools enroll.

Republicans have long wanted to turn this program into a voucher. Instead of money going to schools based on the composition of their student body, Title I would “follow the child.” Every disadvantaged student a school enrolled would come with a small pile of federal cash to help pay for his or her education. And schools would get the money whether they were public, private, or charter.

This idea, known as “Title I portability” in education circles, is by now a mainstream Republican policy proposal. Ronald Reagan called for turning Title I into vouchers during his presidency. Mitt Romney wanted to turn both Title I and special education funding into vouchers during his 2012 presidential run. Sen. Lamar Alexander, the Tennessee Republican who chairs the Senate education committee, introduced a budget amendment to turn Title I into a voucher that could be used at private, public, or charter schools in 2013. (Alexander later supported a narrower version of portability that would apply only to public and charter schools.)

Title I money is meant to help schools that face the challenge of educating a lot of poor students; making it portable means some federal money would also go to schools that are generally wealthy but enroll a handful of kids from poor families. But turning Title I into vouchers wouldn’t be enough on its own to start a private school exodus. Dividing $14 billion in federal funding among the 25 million students poor enough to count as “disadvantaged” yields a voucher of $580. Private school tuition costs, on average, nearly $11,000 per year.

Trump’s plan, though, wants to go a lot bigger. He’d limit the vouchers to students in poverty (which would mean that 14 million kids from families making between 100 and 185 percent of the federal poverty line would be cut out). He’d cut other federal programs besides Title I to bring the funding to $20 million, though he hasn’t said which ones.

But the big idea Trump is touting is that he could get states to kick in enough money to give the vouchers some real buying power.

Trump thinks states will spend $110 billion on vouchers

Trump’s plan calls for turning education spending into a grant to states — and then using that money to encourage states to pass voucher-friendly laws and kick in money of their own. If states added $110 billion of their own money to the $20 billion the federal government would spend, Trump says, every student living in poverty could get a $12,000 voucher, well over the average cost of private elementary school tuition and slightly under the average cost of private high school.

But that’s a little bit like saying if every state somehow passed a law to give students a Porsche, every student would have a Porsche. Trump’s plan is vague on where that $110 billion — 20 percent of the total amount states spend on K-12 education — would come from, or how exactly he’d persuade states to change their laws and contribute the money.

Just 13 states and Washington, DC, have voucher programs at all, and most of those programs only apply to students with disabilities. Seventeen states allow residents or businesses to donate a portion of their taxes to private school scholarship funds, a sort of backdoor voucher system because it allows for some public subsidies for private education.

Given that Republicans have total control of state government in 25 states, more states might be voucher-friendly soon. Although some blue states would certainly resist, dangling federal grants as a reward can lead to dramatic change. The Obama administration is proof: In 2009, as states were struggling with their budgets during the recession, a state grant competition, Race to the Top, successfully got dozens of states to change their laws on charter schools and adopt Common Core standards in order to be eligible for grants totaling $4 billion.

Republicans spent the next seven years decrying Obama’s Race to the Top program as federal overreach. And some school voucher supporters, including the libertarian Cato Institute, are arguing that if Trump wants to use the Education Department to urge states to embrace vouchers, he’d be no better. Congressional Republicans will have to pick between two principles they hold dear: school vouchers and states’ rights.

But even if Trump got a majority of states to pass school vouchers, the evidence suggests that students who took them might not get a dramatically better education.

Some research — but not all — has found school vouchers work

School vouchers are particularly popular among Republicans because they appeal to two core constituencies — free market conservatives and evangelical Christians — while also allowing them to use the language of civil rights and social justice.

Giving school vouchers to 11 million children would be a huge windfall for private schools — the majority of which are affiliated with religious groups. Religiously affiliated private schools outnumber secular ones two to one nationally. Conservative Christians have supported vouchers since the 1970s, after desegregation and Supreme Court decisions on school prayer and Bible reading led to a backlash against public schools.

Voucher advocates, though, also argue that school vouchers give poor families a right that middle-class ones take for granted: the ability to choose the school their children attend. The majority of children in America go to a public school they’re assigned to based on their address.

But well-off families can afford to pick out a house zoned into the public school they want their kids to attend, while poor families with fewer resources are stuck with the neighborhoods they can afford. (Many Democrats embrace this argument too, but only when it refers to charter schools rather than school vouchers.)

This argument, though, raises the question of whether private schools really offer a better education to voucher students. And two decades of research have suggested the answer to the first question is “maybe, but not by much” — at least if you’re measuring quality by test scores.

Most voucher programs in the US have been small and awarded by lottery, meaning researchers can compare students who wanted vouchers and got them with students who wanted vouchers and didn’t get them. That’s generally considered the best way to measure the effect of a private school education, since you’re only comparing students who wanted the vouchers in the first place. (Some students who are awarded vouchers end up attending public school anyway; researchers have tried to compensate for this.)

Two decades of studies on voucher programs in six cities and two states have not led to a resounding conclusion that vouchers work to improve education across the board. Of the 20 studies by the pro-voucher Friedman Foundation for Educational Choice, 10 found vouchers had no effect on participants’ test scores at all.

In Toledo and Dayton, Ohio, vouchers had no effect on students’ test performance. In DC, students who won the voucher lottery were more likely to graduate from high school but didn’t score any higher on state tests after three years in private school. In Charlotte, North Carolina, vouchers appear to have led to higher test scores in reading, but not math, for participants. In Milwaukee, voucher recipients made faster progress in math.

New York’s voucher program, meanwhile, has been intensely studied and debated. Most analyses agree that it had no effect overall on test scores or college enrollment rates. But other research found that black students who got vouchers later performed better on tests, even if students of other races saw no effect — which led to a debate among researchers about how students should be classified, because if students with a black father and a mother of another race were included (typically, research assigns children their mother’s race), the effect disappeared.

On the whole, while some studies have found a modest improvement in test scores, particularly for black students, there’s far from a resounding consensus from studies of citywide voucher programs.

Meanwhile, recent studies of newer statewide voucher systems in Louisiana and Ohio found that students who used vouchers actually fell behind academically, particularly in math. The pro-voucher Fordham Foundation, which analyzed Ohio’s program, was honest that it found those results dismaying: “We did not expect — or, frankly, wish — to see these negative effects for voucher participants,” the researchers wrote.

The assertion that vouchers push public schools to improve by providing competition, on the other hand, has a somewhat more solid foundation. In Louisiana, students who got vouchers but chose not to use them saw their test scores rise, suggesting that public schools might be improving in response to competition. An analysis of 21 separate papers found that, overall, vouchers had either no effect or a slight positive effect on surrounding public schools. (Many of those studies were done in Florida, where vouchers were part of a complex, high-stakes system meant to increase schools’ average test scores.)

Trump is proposing school vouchers on an unprecedented scale. It’s not clear how these effects would translate if 20 percent of the nation’s public school population had the option of transferring to a private school.

The worst-case scenario: Bad private schools proliferate, and public schools get worse

Trump’s school vouchers, if they took effect, would be far more extensive than any existing voucher program in the US. And it’s not clear what quality control, if any, Trump envisions — if private schools enrolling voucher students would have to give standardized tests and be judged according to their scores, for example.

DeVos, Trump’s education secretary pick, pushed to expand charter schools in Michigan — a state that lets for-profit companies run charter schools and has some of the weakest regulation in the country. And her group campaigned against a state law that would have created oversight for Detroit charter schools.

In higher education, where the federal government provides billions of dollars of vouchers (known as Pell Grants) for poor students, plenty of that money goes to colleges where students never graduate or earn degrees that can’t get them a job.

By covering only families below the poverty line, Trump’s plan would also leave many students, including disadvantaged students, behind in public schools. That would likely include some students who received vouchers but struggled with private school admissions. Private schools can keep their admissions standards, meaning they can reject students based on a variety of factors, including their academic and disciplinary background.

In Ohio, for example, students who used vouchers were slightly less disadvantaged than the students who received vouchers and stayed in public schools. The researchers suggested this might be because getting admitted to a private school requires both the ability to meet admissions requirements and the savvy to navigate the system.

A child living in poverty in a city who nonetheless has good grades and test scores, has no disciplinary record, and doesn’t mind attending a Catholic or conservative Christian school would have options under a voucher system. It’s not clear what Trump’s plan would do to ensure they’re good ones.

A child with learning disabilities, or who is still learning English, or who lives in a rural area with few private school options, or who has been suspended from school — a punishment disproportionately applied to black students — will have a lot fewer. And it’s even less clear what will happen to the public schools she might end up attending instead.