As pollsters and data analysts struggle to explain how Sen. Bernie Sanders (I-VT) overcame a 30-point-plus deficit to defeat Hillary Clinton in the Michigan primary on Tuesday, one possible factor that has been neglected is the “reverse Bradley effect” — the possibility that black voters were reluctant to tell pollsters they opposed Clinton.

The “Bradley effect” refers to the theory that white voters might lie to pollsters about supporting a black candidate, then vote against that candidate in the privacy of the voting booth. It is thought to explain Tom Bradley’s surprise defeat in the 1982 race for California governor against George Deukmejian — though the Deukmejian camp points to his big lead in the absentee vote as the real reason, since Bradley, L.A.’s first black mayor, won on Election Day.

In 2008, there was much debate over whether the “Bradley effect” might cause Barack Obama to under-perform in polls as nervous white voters secretly voted against him. (The effect failed to materialize, and Obama won handily.)

In Michigan, polls consistently showed Clinton with a big double-digit lead against Sanders, right up until the last week before the primary. There were no obvious controversies about remarks the candidates made in the last two debates, which were held by CNN on Sunday evening and by Fox News on Monday afternoon (though Fox grilled Clinton on her emails and on the question of abortion, which previous moderators from other networks had not).

There were no obvious clues in exit polls. One theory for the shift, posed by Carl Bialik of fivethirtyeight.com, is that Clinton voters assumed she would win and crossed over to vote against Trump. But that wave should have showed up more dramatically in the GOP vote.

The only strange data point seems to have been that Clinton underperformed among Michigan’s black voters. After destroying Sanders among black voters in the South — beating Sanders 82.6% to 16.5% in Mississippi on Tuesday, for example, with a 9-to-1 margin among black Democrats — she only beat Sanders 2-to-1 in Michigan. Late returns from Detroit were not enough for Clinton to catch up to Sanders’ statewide lead.

That suggests that black voters may have been under-sampled, or that they may simply have been afraid to tell pollsters that they opposed Hillary, given the Clintons’ reputation for retaliation.

Hence, a “reverse Bradley effect.”

Joel B. Pollak is Senior Editor-at-Large at Breitbart News. His new e-book, Leadership Secrets of the Kings and Prophets: What the Bible’s Struggles Teach Us About Today, is on sale through Amazon Kindle Direct. Follow him on Twitter at @joelpollak.