Dolls are for kids. So why were they in front of the most esteemed judges in the United States?

As they deliberated on Brown v. Board of Education, the landmark 1954 case that eventually overturned “separate-but-equal” segregation in the United States, the Supreme Court Justices contemplated oral arguments and pored over case transcripts. But they also considered baby dolls—unexpected weapons in the plaintiffs’ fight against racial discrimination.

The dolls were part of a group of groundbreaking psychological experiments performed by Mamie and Kenneth Clark, a husband-and-wife team of African American psychologists who devoted their life’s work to understanding and helping heal children’s racial biases. During the “doll tests,” as they’re now known, a majority of African American children showed a preference for dolls with white skin instead of Black ones—a consequence, the Clarks argued, of the pernicious effects of segregation.

The Clarks’ work, and their testimony in the underlying cases that became Brown v. Board of Education, helped the Supreme Court justices and the nation understand some of the lingering effects of segregation on the very children it affected most.

For the Clarks, the results showed the devastating effects of life in a society that was intolerant of African-Americans. Their experiment, which involved white- and brown-skinned dolls, was deceptively simple. (In a reflection of the racial biases of the time, the Clarks had to paint a white baby doll brown for the tests, since African American dolls were not yet manufactured.) The children were asked to identify the diapered dolls in a number of ways: the one they wanted to play with, the one that looked “white,” “colored,” or “Negro,” the one that was “good” or “bad.” Finally, they were asked to identify the doll that looked most like them.

All of the children tested were Black, and all but one group attended segregated schools. Most of the children preferred the white doll to the African American one. Some of the children would cry and run out of the room when asked to identify which doll looked like them. These results upset the Clarks so much that they delayed publishing their conclusions.

Mamie Clark had connections to the growing legal struggle to overturn segregation—she had worked in the office of one of the lawyers who helped lay the foundation for Brown v. Board of Education. When the NAACP learned of the Clarks’ work, they asked them to participate in a case that would later be rolled into the class-action case that went to the Supreme Court. So Kenneth Clark headed to Clarendon County, South Carolina, to replicate his test with Black children there. It was a terrifying experience, he recalled later, especially when his NAACP host was threatened in his presence.

“But we had to test those children,” he recalled. “These children saw themselves as inferior and they accepted the inferiority as part of reality.”

Thurgood Marshall was eager to use the Clarks’ work in the bigger class-action case that would become Brown v. Board of Education, but not everyone was convinced. Attorney Spotswood Robinson told an observer that it was “crazy and insulting to persuade a court of law with examples of crying children and dolls,” writes historian Martha Minow.

But the court didn’t think so. Kenneth Clark testified at three of the trials and helped write a summary of all five trials’ social science testimony that was used in the Supreme Court case. He told judges and juries that African American children’s preference for white dolls represented psychological damage that was reinforced by segregation.

“My opinion is that a fundamental effect of segregation is basic confusion in the individuals and their concepts about themselves conflicting in their self images,” he told the jury in the Briggs case. The sense of inferiority caused by segregation had real, lifelong consequences, he argued—consequences that started before children could even articulate any information about race.

The Clarks’ work and testimony were part of a much broader case that combined five cases and covered nearly every aspect of school segregation—and some historians argue that the doll tests played a relatively insignificant part in the court’s decision. But echoes of the Clarks’ results ring through the unanimous opinion of the Supreme Court justices.

“To separate [Black children] from others of similar age and qualifications solely because of their race generates a feeling of inferiority as to their status in the community that may affect their hearts and minds in a way unlikely ever to be undone,”wrote Chief Justice Earl Warren in the opinion. The Clarks’ work had helped strike down segregation in the United States.

Today, one of the Black dolls is on display at the Brown v. Board of Education National Historic Site in Kansas, and integration is the law of the land. But the racial biases the couple documented in the 1930s and 1940s still exist. In 2010, CNN commissioned an updated version of the study using cartoon depictions of children and a color bar that showed a range of skin tones—and found results that were strikingly similar to those shown by the Clarks.

In the new test, child development researcher Margaret Beale Spencer tested 133 kids from schools with different racial and income mixes. This time, the studies looked at white children, too. And though Black children seemed to hold more positive views toward Black dolls, white children maintained an intense bias toward whiteness.

“We are still living in a society where dark things are devalued and white things are valued,” Spencer told CNN. Jim Crow segregation may no longer exist in the United States, but racial bias is alive and well.