Parents often assume that if cell phones interfere with learning, it's because children are distracted by texts, games or social media during class.

The problem may run deeper. According to a recent study, elementary school students with their own cell phones scored significantly lower on reading comprehension tests than classmates who did not own one, even when the phone wasn't being used in school.

A University of California research team wanted to answer an important question that parents, educators and school districts continue to debate: Do cell phones hurt learning because they distract students during class or does simply owning a phone influence academic performance?

Children who had their own cell phones consistently scored lower on reading comprehension than children who did not have one. That turned out to be true in third grade and in sixth grade.

To find out, the investigators studied 106 elementary school children from culturally and linguistically diverse public schools.

The participants included 55 sixth graders and 51 third graders. Sixty of the children owned their own cell phone, while 46 did not. Each child completed reading comprehension tests under different conditions. Students who brought their phones to school took one test with the phone sitting next to them on the desk and another with the phone put away.

The researchers, led by Patricia Greenfield, Distinguished Professor of Psychology at the University of California, Los Angeles, along with co-author Merola Hanna, a UCLA postgraduate research assistant, and others also collected information about phone ownership, age at which children received their first phone and texting habits.

The result was surprising: having a phone nearby during testing made virtually no difference. Reading scores were almost identical whether the phone was sitting on the desk or stored away. In other words, simply seeing the phone wasn't enough to interfere with performance. Instead, the strongest finding involved ownership itself.

Children who had their own cell phones consistently scored lower on reading comprehension than children who did not have one. This was true both in third grade and in sixth grade.

Elementary school students with their own cell phones scored significantly lower on reading comprehension tests than classmates who did not have one, even when the phone wasn't being used in school.

"We found that there's no age at which you can give a child a phone that's better or worse for reading comprehension, so it's just having a phone at all," Patricia Greenfield said in a news release.

The researchers cannot say with certainty why this relationship exists because the study was designed to identify an association, not prove cause and effect. But they do believe phone ownership may gradually change how children spend their free time. Hours that might once have been devoted to reading books may instead be spent texting friends, playing games or scrolling through online content.

Over time, less sustained reading could make it harder to develop the comprehension skills needed to understand increasingly complex material.

One encouraging finding emerged for bilingual children. Among students from homes where English was not the primary language, those who began texting at younger ages actually performed better on reading comprehension than bilingual classmates who started texting later. The researchers speculate that texting may provide additional opportunities to practice English vocabulary and communication skills.

Over time, having less experience with sustained reading could make it harder to develop the comprehension skills needed to understand increasingly complex material.

The findings do not necessarily mean parents should panic or immediately ban all cell phone use. The study was relatively small, involved just over 100 children from one geographic area, and the researchers note that longer term studies are needed to determine whether phone ownership directly affects reading development or whether other factors also contribute.

Still, parents may want to think carefully before giving elementary school children their own cell phone. While school cell phone bans may reduce classroom disruptions and encourage more face-to-face interaction, this study suggests that what children do with a phone outside school may matter even more than whether it's sitting in a backpack during math class.

The study is published in Behavioral Science.