At first blush, black voters appear to be an almost monolithically Democratic bloc. In 2016, black Americans cast 24 percent of Democratic primary votes — the largest share ever. And in the general election, 89 percent of black voters supported Democratic nominee Hillary Clinton. That’s one of the reasons why South Carolina’s primary on Feb. 29 is seen as a bellwether for the Democratic field — if a candidate can’t win a state where a majority of Democratic primary voters are black, what does that mean for his or her candidacy going forward?

But black voters aren’t the monolith exit polls make them out to be. Pew Research Center found that a quarter of black Democrats identify as conservative, and 43 percent identify as moderate.

So how to square that circle? How can a big chunk of black voters be unwavering Democrats who differ ideologically from the party? We spent years investigating that question for our new book, “Steadfast Democrats.” We found that black voters are so loyal to the Democratic party in part because of social pressure from other black voters. Rather than throw the whole book at you, we’re going to highlight a couple pieces of evidence that show how that dynamic works.

Our first piece of evidence came from survey data collected by the 2012 American National Election Study (ANES). In that survey, interviewers asked respondents face to face which party they identify with. We then looked at the race of the interviewer and the race of the respondents to see if black respondents generally answered differently depending on who asked the question. We concluded black respondents were more likely to report they were a Democrat when they were with a black interviewer (96.4 percent) than a nonblack interviewer (83.9 percent) or an online survey (85 percent).

We ran a separate study around the 2012 presidential election to test the same theory. We wanted to determine the likelihood that black individuals would defect from the norm (supporting Democratic candidates) when offered money. In the study, 106 black students at a midwest college were randomly assigned to one of three groups. Each was given $10 by an interviewer and told the money could be donated to Mitt Romney or Barack Obama. Subjects were informed that they were not obligated to donate the money and that they could decide to keep it. But if they chose to give it to a candidate, $10 would be donated for every $1 they allocated. (This was just a ruse — the money was not actually donated.) They were also told that they should make their decision once they entered a separate room, away from the interviewer, where there would be one contribution box for Romney and another for Obama.

But not all the students were in the room alone. One group of students was, but two other groups were paired with an actor pretending to be another participant. In each scenario, the actor was instructed to walk into the room and immediately say out loud that he or she was donating all the money to Obama, then make the donation. One group paired participants with a white actor; in the other, the actor was black.

People in the first group — the loners — kept most of the money, donating on average $3.74 to the Obama campaign. In the group with the white actor, individuals donated $4.45 to the Obama campaign. This amount was not statistically different from the scenario in which no actor was present. But in the third group, with the black actor, the average Obama contribution increased to $6.85 — a significant increase relative to the group where no actor was present.

In other words, black participants were less likely to pocket the money when another black person said he or she would be donating to Obama. The participant felt pressure to comply with the expectation of behavior by someone similar to them.

As you see the results from South Carolina and other states with a percentage of black Democrats, keep that dynamic in mind. The values of the Democratic party appeal to many black voters, but their steadfast loyalty to the party goes beyond common interest. Social pressure is what cements that relationship between the black electorate and the Democratic party.