ABUJA, Nigeria—Masturah Musa kneads a ball of halawa with her fingers. As it begins to soften, she spreads the sticky caramel across her customer’s leg, then pulls it upward.

Over the summer, researchers published a study that offered proof of a phenomenon in American black communities that has existed since slavery: By being perceived as more mature, black girls fall victim to what researchers are calling a “perception trap,” and are treated negatively as a result.