Although opinion polls currently put President Donald Trump’s approval among black voters at between 20 and 30 per cent, most experts doubt that anything like that proportion of the black vote will go to Trump on Election Day.

Yes, strange things happen in politics, remember the “blue wall”?, but the “black wall” of overwhelming support of black voters for Democratic candidates seems impermeable.

Everyone knows the wall exists. Everyone takes it for granted. Yet on issues from school vouchers to abortion, significant numbers of black voters are closer to the Republican than the Democratic point of view. Yet they still vote Democratic. Why?

Most answers verge towards the condescending and simplistic, “oh, all Republicans are racist”, as though the black monolith could not possibly comprise a politically diverse community whose members might judge for themselves the salience of various issues.

What’s needed is a thoughtful, scholarly treatment of exactly why black voters remain with a Democratic party that, for many, has moved a good distance from what they most deeply value.

Happily, that gap has been filled by a new book, Steadfast Democrats: How Social Forces Shape Black Political Behaviour, by political scientists Ismail K. White and Chryl N. Laird — a volume that everyone interested in American politics should be reading.

White and Laird treat the propensity of black voters to support Democratic candidates not as an eternal verity but as a puzzle to be solved. They point out that although in other ethnic groups, whites, Asian-Americans, Hispanics, conservative views are a good predictor of voting Republican, among black Americans they aren’t.

The authors acknowledge that the pattern can’t be explained by Democratic support of civil rights laws, because the tendency significantly predates those laws.

In fact, as I’ve shown elsewhere, during the 1940s the Republicans were the more progressive party on civil rights. Black voters left anyway.

A month before the 1944 election, Republican presidential nominee Thomas Dewey gave a speech about racial equality. A furious John W. McCormack, the House majority leader and the second-most-powerful Democrat, responded by calling him “reprehensible” for injecting “racial issues into this campaign”.

Dewey built much of his campaign around civil rights laws, but black voters continued to flock to the Democrats.

Whatever the historical cause, White and Laird posit as an explanation for today’s puzzle what they call a “racialised social constraint”, pressure on black voters from black peer networks to adopt a partisan Democratic outlook.

During the 2012 campaign, for example, black voters were four times as likely as white voters to report that they expected sharp criticism from friends and family if they voted Republican. This phenomenon was not attributable to the fact that Barack Obama, the nation’s first black president, was on the ballot: in 2016, black voters expected disapproval at about the same rate as in 2012.

Here’s another striking result: the larger the proportion of a black person’s friends who are also black, what’s known as high racial homophily, the more likely he or she is to vote Democratic.

For black voters who have a majority of non-black friends, however, the chance that he or she will vote Democratic drops to a coin flip.

Among white voters, on the other hand, the link between racial homophily and voting behaviour “is essentially zero.”

Small wonder, then, that during the 2012 campaign, when Obama ran for reelection against Mitt Romney, about 45 per cent of white Democrats reported being urged by friends or family to vote for Obama (as opposed to a general exaltation to vote). Among black Democrats, the figure was just shy of 74 per cent.

White and Laird conducted several experiments that provide striking support for their thesis. In a particularly telling 2012 study, the authors discovered that black college students given a sum of money and offered the chance to contribute some of it to the Obama campaign gave nearly twice as much as the black control group when they knew that other black students would know how much they gave.

On the other hand, when the only observers were white students, the difference with the control group was not statistically significant.

As the authors explain, social pressure isn’t everything, and black voters are no less rational in pursuing their electoral interests than anyone else.

They simply calculate their interests differently, responding not only to the policy positions of candidates but also to their concerns about what other black people will think of them. The judgment of your social network matters a lot more when you’re part of the out-group.

White and Laird seem not entirely disheartened by their findings. They argue that “a group’s ability to define its boundaries is essential to its existence and its ability to mobilise”. Yet their research suggests a contradiction.

What they really show is the existence of more than one black “group”, bound together by a political solidarity that would not exist were it not enforced by racialised constraints.

As the authors concede, it’s not clear whether the constraints can long survive the decline in influence of the institutions that once ruled the black community, the churches, the social clubs, the colleges, a decline that itself stems in part from integration.

This concession matters. I doubt that Trump will be the Republican who breaches the “black wall”, but there’s no reason to imagine that it will stand for ever.

That’s why Democrats, if they’re smart, will set about building a bigger tent on such issues as school vouchers. Because if they wait until the wall crumbles, and it will crumble, they’ll be too late.

• Stephen L. Carter is a Bloomberg Opinion columnist. He is a professor of law at Yale University and was a clerk to US Supreme Court Justice Thurgood Marshall