One July afternoon in 1972, a team of psychologists took sound meters into the Bridge Apartments, a cluster of four high-rise buildings straddling Interstate 95 in Manhattan.

Because of the towers’ proximity to the highway, the hum of traffic filled the buildings’ halls. Even on the eighth floor, the decibel level was 66, just slightly quieter than a running vacuum cleaner. The noise died down as the researchers climbed the stairs, though. On the 32nd floor, the reading was 55 decibels, or about the level of a conversation in a restaurant. Decibels are exponential, so this meant the lower floors were about six to 12 times louder than the upper floors, as Colleen Moore, a psychology professor at the University of Wisconsin, explains in her book, Silent Scourge.

The researchers, led by Sheldon Cohen, then with the University of Oregon, discovered something interesting about the children of the families living in that building. School-aged kids living on the lower, noisier floors had more trouble hearing the difference between two similar words, such as thick and sick, than those on the upper, quieter floors.

The children on the noisier floors were also worse at reading. The relationship between the kids’ scores and floor level was strongest for the kids who had lived in the building the longest.

In their study, Cohen and his colleagues speculated that the children on the lower floors might have learned to filter out the background noise, but their ability to differentiate among words suffered in the process. “A case is gradually emerging for the stressful impact of noise on behavior,” they wrote in 1973.

That case has been bolstered by some recent studies showing that children who were taught new words in quiet environments were later able to say them more accurately. Similarly, in a study published in Child Development on Thursday, University of Wisconsin researchers Brianna T. M. McMillan and Jenny R. Saffran found that background noise makes it harder for toddlers to learn new words.

For the study, 106 children aged 22 to 30 months took part in three experiments. First, a group of 2-year-olds were taught two nonsense words (things like “tursey” and “blicket”) in the presence of background noise that was either 5 or 10 dB quieter than the voice of the teacher. The children successfully learned the words when the background noise was quiet, but not when it was loud. The same thing happened when the researchers tried the test with a slightly older group of toddlers.

In a third experiment, McMillan and Saffran found the toddlers could learn the meanings of new words in a noisy environment—but only if they had been introduced to those words in a quiet setting first.

“Hearing new words in fluent speech without a lot of background noise before trying to learn what objects the new words corresponded to may help very young children master new vocabulary,” Saffran said in a statement.

That’s a helpful tip for parents and teachers, but overall, the study highlights yet another cognitive obstacle facing low-income children. Not only do poor children hear fewer words than rich ones—the gap is estimated to reach 30 million words by age 3—they are more likely to live in loud environments, as McMillan and Saffran write. Their homes are more crowded, their schools are closer to highways, and they spend more time watching TV. (This phenomenon would help explain why children living in urban poverty have lower verbal working-memory scores than those in rural environments.)

If the connection between noise and learning bears out in future studies, it will underscore yet another way that being poor hurts the body and mind.