Why are black preschoolers in America more than three times as likely to be suspended than their white classmates?

Perhaps because teachers are more likely to expect young black children — especially young black boys — to misbehave, according to a new Yale study.

The study, conducted by researchers at the Yale University Child Study Center, asked more than 130 preschool teachers to watch video clips of children in classrooms. The teachers were told to look for signs of “challenging behavior.”

The children in the videos were actors, and the clips did not actually show any challenging behaviors. But the teachers didn’t know that. They were anticipating trouble. And as they scanned the video clips, looking for signs of that trouble, they spent more time looking at black children than white children, according to equipment that tracked their gaze.

The teachers spent even longer looking at black boys.

That’s a sign that teachers expect problems from black children, and especially black boys, said lead researcher and Yale child psychology professor Walter S. Gilliam. It’s a finding that shows how deeply rooted racial biases are, he said, and how badly teachers need training to confront and unravel the knee-jerk perceptions of their students — perceptions they often don’t even realize they have.

“Implicit biases do not begin with black men and police. They begin with black preschoolers and their teachers, if not earlier,” he said, referring to the multiple fatal shootings of black men by police that have given rise to the Black Lives Matter movement and a national debate about law enforcement’s treatment of people of color. “Implicit bias is like the wind: You can’t see it, but you can sure see its effects.”

Black children accounted for 19 percent of all preschool students in 2013-2014, but they made up 47 percent of those who received suspensions, according to federal civil rights data.

The study also asked teachers to complete a second task: Read a vignette about a student misbehaving in class, then rate the severity of misbehavior and decide whether the misbehavior warranted suspension, expulsion or neither.

Researchers told some of the teachers that the child’s name was DeShawn or Latoya, stereotypical black names; others heard that the child’s name was Jake or Emily, stereotypical white names. Again, researchers found that teachers’ responses differed by race: White teachers were more lenient on children they perceived as black, while black teachers were harsher.

Teachers weren’t asked to explain their ratings. But researchers said that the racial differences in their response are consistent with the theory that white teachers see black preschoolers as more likely to misbehave, so they don’t see a black child’s misbehavior as severe.

Some teachers received background information about the child’s difficult family life, to test whether such additional information might spur a more empathetic response. The empathy kicked in only when the teacher and the child shared the same race, the study found.

Linda K. Smith, who coordinates policy for Head Start in the Obama administration’s Department of Health and Human Services, said that the study offered a tough but important message for the field of early childhood education, given its roots in social justice and the notion that all children have great potential.

Only one of the 135 teachers involved in the study asked to withdraw her data after learning the real purpose of the research. Smith said that is a sign that early childhood educators are committed to the uncomfortable job of facing their own biases.

“It’s something probably we all didn’t want to hear, but we needed to know,” she said.

The study was funded by the W.K. Kellogg Foundation and is expected to be released Wednesday at a meeting of state Head Start administrators.