The clash between Joe Biden and Kamala Harris over desegregation busing programs that Biden opposed in the 1970s brought more heat than any other single incident across four hours of Democratic debating.

From a policy standpoint, their argument raised more questions than it answered. After all, at the end of the day, the busing fight — which in its most ambitious versions involved sending students from black urban neighborhoods to schools in white suburbs and vice versa — is one that its foes won pretty decisively. Nobody in American politics, including Harris, seems to be calling for a return to the sort of federally mandated integration busing programs that Biden fought. Even if they were, the Supreme Court’s Parents Involved decision in 2007 put significant constraints on even voluntary district busing programs, and the Court has only grown more conservative since then.

But the exchange came amid a renaissance of interest in integration over the past few years from progressive education thinkers and civil rights activists — and 2020 candidates seem to be taking notice. Harris’s team, meanwhile, is suggesting that she supports busing in some form but hasn’t clarified what exactly she means by that.

Bernie Sanders, meanwhile, has introduced what Century Foundation senior fellow Richard Kahlenberg tells me is “the most robust school integration plan of any of the candidates” — an agenda that, if enacted, would be by far the most significant pro-integration step the federal government has taken in a decade.

Former Housing and Urban Development Secretary and San Antonio Mayor Julián Castro has his own integration ideas, fittingly focused more on the housing side of things. And Elizabeth Warren is a co-sponsor (along with Sanders) of the Strength in Diversity Act, the leading congressional vehicle for school integration, spearheaded by Sen. Chris Murphy (D-CT), and has in the past supported de-linking school admission from geography entirely, though her campaign is cagey on whether she still thinks that’s a good idea.

By some measures, de facto school segregation is on the rise

Harris opened her salvo by noting that Biden had spoken fondly of working with segregationist senators on issues, then segued into the observation that one of the issues he’d worked with them on was opposing federal busing programs — something she said she found personally hurtful as someone who benefited from such programs as a young girl. Biden in response relied on a 1970s-vintage policy distinction between de jure and de facto school segregation.

The meaning of this was that much of the United States, most notably the entire South, had a longstanding policy of de jure school segregation into the 20th century — it was formally illegal for an African American student to attend a white school. It didn’t matter where you lived or what taxes you paid; some schools were for white students and others were for black students.

Such policies were deemed unconstitutional by the Supreme Court in its landmark Brown v. Board of Education decision in 1954, but because school administration is a fundamentally local function in the United States — and was even more local back then — the decision did not itself automatically integrate schools. Instead, integration of de jure segregated schools required successive rounds of court orders, actions by the federal executive branch, and at times the actual cooperation of state governments.

As late as 10 years after Brown, there was almost no actual integration in the South, but in the 10 years following the passage of the Civil Rights Act in 1965, the number of black students attending majority-white schools soared and integration was actually happening.

Then litigation began to turn toward de facto segregation and, crucially, to the North, based on a 1971 Supreme Court precedent from North Carolina that ruled that Charlotte’s formerly de jure segregated school system had to achieve actual racial mixing in its schools, even if that meant assigning students on a non-geographical basis. That legal reasoning proved to be a conceptually powerful lever for attacking de facto segregation in the North.

A common situation in Northern cities was that there was no rule against black kids attending a white school. There was, instead, a long legacy of policy decisions (restrictive covenants, redlining) and informal norms that had created a stark pattern of residential segregation. School assignment boundaries were then drawn to reinforce the underlying pattern of residential segregation and create a situation where, de facto, the schools were segregated, even if there was no law preventing black students and white students from attending school together.

Wilmington, Delaware, was one of those cities, and Biden’s legacy on this issue was shaped by concerns that courts would order the busing of black kids from black neighborhoods of Wilmington into the white suburbs and vice versa.

An optimist 40 or 50 years ago might have hoped that de facto segregation would simply fade away over a generation or two, as the formal policies that undergirded de jure segregation were no longer there to support it.

But this hasn’t really happened. Measuring segregation across time is difficult because the underlying demographics have changed so much — these days, white kids are a much smaller share of the school-age population and Latino and Asian kids are a much larger one — but depending on how you look at it, progress on de facto desegregation has either stalled or reversed.

Attending segregated schools seems to be harmful

If the negative outcomes of segregation were simply the consequences of minority families being poor, which is independently bad, that’s something you would probably want to address directly by helping low-income people have more money rather than by putting their kids on buses.

But the balance of evidence suggests that segregation, full stop, really is harmful to black students. A 2006 study from David Card and Jesse Rothstein, for example, suggests that full de facto integration of schools might close a quarter of the black-white SAT score gap even while holding all other differences in background conditions constant.

Jacob Vigdor and Jens Ludwig found in 2007 that essentially all the harms that come from growing up in a segregated neighborhood are attributable to the fact that this means you will very likely attend a segregated school.

White students who attend heavily black schools, meanwhile, do perform somewhat worse on average than white students in very white schools. But statistically speaking, this is entirely attributable to the fact that these white students’ parents are poorer and less educated than the average white parent — with statistical controls for those factors, there’s no ill effect. Black students, on the other hand, do benefit from attending more diverse schools.

None of this is to say that segregation is necessarily an insuperable obstacle to educational excellence. Critics often note that the charter school sector is highly segregated, while proponents argue that many charter schools offer excellent educational opportunities. These are not, however, necessarily contradictory assessments. In DC’s Shaw neighborhood, where I live, the local KIPP elementary school is 0.7 percent white and 95 percent black, while DCPS’s Garrison Elementary School is 15 percent white and 49 percent black. KIPP’s students, however, are doing significantly better on the city’s assessments of educational performance.

But by and large, the evidence seems clear that generating a more integrated school system would be a good idea — if you can pull it off.

White parents resist integrated schools

The biggest problem of school integration is that white parents mostly don’t want their kids to go to schools that lots of black students attend. Liberal whites on the Upper West Side of Manhattan went into mass panic mode over a school integration plan in 2018; a semi-accidental influx of black students into a white school in the suburbs of St. Louis in 2015 provoked a massive backlash; and a major trend in recent years has been for affluent areas to secede and form their own school districts to preserve homogeneity.

So while many different institutional arrangements, some of which involve buses and some of which don’t, could theoretically generate racially mixed classrooms, in practice they tend not to, because white parents mostly do what they feel they need to do in order to avoid the integrated outcome. A critical piece of this is that whether or not a given school district is formally organized around some form of school choice paradigm, in a practical sense, parents — and especially high socioeconomic status parents — exert choice over their kids’ schooling when they choose where to live.

As one of the parents in the St. Louis suburb upset by the new cohort of black students put it, “I shopped for a school district — I deserve to not have to worry about my children getting stabbed or taking a drug or getting robbed.”

Parents, in general, want their kids to attend “good schools.” But while it might be nice from the standpoint of education wonks for parents to assess school quality on the basis of detailed examination of instructional performance, in practice, few people do that.

New York City has adopted an open enrollment lottery to assign kids to high schools, which allowed Atila Abdulkadiroğlu, Parag A. Pathak, and Christopher R. Walters to study whether parents value sophisticated measures of school performance when deciding which high schools to apply to. They conclude that they do not. Instead, “parents prefer schools that enroll high-achieving peers.”

Meanwhile, Chase M. Billingham and Matthew O. Hunt show that beyond using race as a crude proxy for quality, some white parents just prefer that their children attend schools with fewer black students. Further exacerbating segregation is evidence, as found by Justine Hastings, Thomas Kane, and Douglas Staiger in their study of school lotteries in the Charlotte-Mecklenburg school district, that black parents preferred schools that are blacker than average.

As a result of all this, an idea that sounds like it would work — San Francisco’s program to let parents pick whichever school they want regardless of neighborhood — has failed in practice. As Dana Goldstein reported for the Times back in April, the rollout of unrestricted choice actually ended up making the city’s schools more segregated.

Of course, a city can try to instead take a more deliberate approach. But the Supreme Court’s 1974 Milliken v. Bradley ruling established that federal courts couldn’t order desegregation busing across school district lines. That sharply limited the prospects for integration in metro areas that — like Detroit, where the case originated — featured a heavily black central city surrounded by heavily white suburbs. But it’s also relevant to more diverse cities like Washington, DC, that nonetheless have segregated school systems.

The city’s majority black electorate and mostly African American political establishment could force the city’s white residents into a school desegregation initiative. But DC officials have told me they fear the practical upshot of such a program would be to drive white flight out of the neighborhoods with majority-white schools, eroding the city’s tax base and leaving black students worse off than before. Elsewhere, affluent neighborhoods have begun a trend of seceding from the surrounding school district in order to create a new, richer, less diverse school district that is immune to integration pressures.

Meanwhile, the 2007 Parents Involved case held that it’s unconstitutional to explicitly use race as a factor in school assignment within a district, even if integration is the intended purpose. The result is that while, conceptually, at least, it’s easy to generate integrated schools, the practical barriers to doing so are very high.

Consequently, the new thinking on this focuses on monetary carrots rather than judicial sticks — aiming to create a situation where the positive incentive to integrate is stronger.

Integration carrots rather than sticks

The core provision of the Murphy/Fudge Strength in Diversity Act is to provide $120 million to support districts (or groups of districts) that want to undertake voluntary integration measures.

In a practical sense, the amount of money on hand here probably wouldn’t make much of a difference. But in light of all of the above, the underlying philosophy seems fairly sound. Most Americans are broadly supportive of school integration in the abstract, even if individual white families’ choices are the reverse. Meanwhile, virtually everyone likes lower taxes and better-funded local schools. Putting extra money on the table could make white families less resistant to attending blacker schools. It could also reassure districts that it’s worth paying the financial price of accepting a certain amount of white flight in order to pursue integration measures.

Sanders’s “Thurgood Marshall Plan for Education” incorporates these ideas, but would also go further by repealing the existing ban on using federal transportation funding to promote school integration and create a $1 billion fund to support magnet schools as an additional desegregation lever. Beyond legislation, Sanders is also promising to “execute and enforce desegregation orders and appoint federal judges who will enforce the 1964 Civil Rights Act in school systems,” which appears to be a promise to try to revisit the substance of the Parents Involved and Milliken cases.

In a practical sense, legislative action on this topic seems extremely unlikely, and not just because of the general difficulty of getting laws passed. Congress reauthorizes the big federal K-12 education law, the Elementary and Secondary Education Act, every decade or so, and last did so in 2015, which means the whole topic isn’t actually ripe for policy change.

So in a practical sense, the renewed interest in desegregation initiatives isn’t particularly timely. Sanders’s decision to roll out his integration initiative is more a token demonstration of his support for desegregation, just as his proposed ban on for-profit charter schools signaled how his overall vision of education policy differs from the “reform” approach of the Obama administration. But the federal judiciary has made an enormous difference on this topic over the years, so the commitment on judges here is potentially very significant.

Meanwhile, there is also the possibility of attacking the residential segregation that undergirds school segregation directly.

Housing policy is education policy

As befits a former HUD secretary, Castro sees school segregation as fundamentally a housing policy issue.

His education platform is extensive but only touches lightly on segregation — calling briefly for “a progressive housing policy that includes affirmative furthering fair housing, implementing zoning reform, and expanding affordable housing in high opportunity areas.” But his proposed housing policies which include making Section 8 housing vouchers available to all families who need it, drastically expanding federal support for constructing low-income subsidized housing, and the first-ever substantial federal effort to get local governments to reform zoning practices that keep lower-income families out of desirable neighborhoods is extremely ambitious.

Warren does not explicitly draw the link to education but likewise has a very ambitious housing policy agenda that in a practical sense would make a huge difference for education.

Back in the early 1970s, activists and courts promoted busing as an integration tool on the plausible theory that it takes a lot less time to map out a new set of school bus routes than to totally revamp a metro area’s built environment.

But rather than busing serving as the fast-acting first step to an ongoing process of desegregation, it turned out to be something of a political dead end. Restarting would, if it can be pulled off, offer the same basic promise as the original busing era — if it works, it works fast — while changing housing is necessarily going to be a slow undertaking.

But the entire concept of de facto school segregation has its roots in patterns of residential segregation — especially in the North, where de jure segregation essentially never existed and racial exclusion always operated primarily through the housing channel.

And on the housing front, there wasn’t even temporary progressive action. Instead, the 1970s are an inflection point at which the scale of exclusionary zoning in the United States started to increase significantly. Housing issues didn’t come up at all in the first debate, but the 2020 cycle is the first one in decades that’s featured candidates with housing platforms— and if the candidates continue to be interested in addressing school segregation, they’d be well advised to come back around to it.