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Toddlers appeared to brush longer on the nights they received more praise from their parents, a University of Pennsylvania study found. Credit: Jackie Mader/The Hechinger Report

It’s a challenge to raise a child who is motivated to do things that aren’t always fun. Why is it that some kids have the persistence to read through the boring passages in a novel, do their math homework or practice piano every day?

Even within a family, some siblings seem to have this magical discipline and others don’t. Those blessed with persistence appear to have an advantage in life because they are more likely to do well in school, graduate from college and enjoy rewarding careers. University of Pennsylvania psychologist Angela Duckworth calls the combination of persistence and passion “grit.”

A team of eight researchers at the University of Pennsylvania are trying to find the keys to motivation and persistence and they’re beginning in an unlikely place: a toddler’s mouth. 

The researchers persuaded 81 parents of three-year olds to take cellphone videos of their children brushing their teeth every night for 16 days during 2019 and early 2020. Cuteness abounded. Pets and siblings intruded. Kids cried.

But the researchers were actually most interested in what the parents were saying off screen. During the nightly brushing, many parents coached their kids to move the toothbrush in circles or explained how to reach their molars. Some parents got creative. One renamed his child’s teeth after football players. “You need to go get Tom Brady in the back!” Another turned brushing into an adventure movie. “Spit out the bad guys!”

All the parents’ utterances were transcribed and categorized as praise, instruction or something else. When the researchers crunched all the data, they noticed that kids brushed longer on nights when they heard more praise. Conversely, toddlers brushed less on nights when their parents gave them more instructions. For example, one child averaged 17 seconds of brushing when the parent didn’t praise them at all. But on a night when the parent gushed with praise, the child brushed for more than 50 seconds.

“This suggests that praise may help children brush longer,” said Julia Leonard, lead author of the study, which she conducted in her previous position as a researcher at the University of Pennsylvania. She is now an assistant professor of psychology at Yale University. “But this isn’t a causal study. It’s just a correlation.”

Like the chicken and the egg riddle, what’s confounding here is what comes first, the brushing or the praise. It could be that children first hear praise and that motivates them to brush even more. Or it could be that kids who brush for a long time prompt lots of genuine praise from their parents. 

“My hypothesis is that it’s both,” said Leonard. “It’s a self-reinforcing process.”

The study, “Daily fluctuations in young children’s persistence,” was published online December 2021 in the journal Child Development. 

Screenshots of adorable three-year-old participants who appear to enjoy brushing their teeth during the study. Photographs printed in “Daily fluctuations in young children’s persistence,” (with parent permission) in the journal Child Development. 

For the dentists out there who might be reading, none of the children was brushing for the recommended two minutes a night. One child averaged five seconds a night. The best brusher averaged 74 seconds a night.

Leonard and her colleagues weren’t really interested in tooth brushing, but they picked it as sort of a pilot test of persistence. “We wanted to capture kids as they were building a habit,”  said Leonard. 

In the future, Leonard hopes to test whether parental praise also influences a child’s motivation to learn to read. But with cognitive tasks, researchers will need to measure a child’s IQ, which also influences a desire to learn. 

It’s important to emphasize that this is a small study of 81 kids, most of whom were white and from higher income families. It’s unclear, from this study, if praise is as motivating for other children. “We know praise differs by culture,” Leonard said. “And so how kids respond to that should very much differ by culture.”

Researchers were also unable to distinguish the effect of different types of praise. Stanford University psychologist Carol Dweck has argued that praising effort is better than praising a person. But, in this study, “Good girl!” was lumped together with “Good job!” and, collectively, all the praise was associated with longer brushing times. 

Researchers also checked to see whether sleep, mood or a parent’s stress level influenced how long a child brushed. On average, these other factors didn’t matter. But for certain toddlers, sleep mattered a lot. That’s inspiring Leonard to think about customized interventions for children. 

Leonard was also struck by how time spent brushing varies so much for each child depending upon the night. One child ranged from a low of five seconds to a high of 102 seconds. Another brusher ranged from 10 seconds to 143 seconds. “Kids have good and bad days,” said Leonard. 

That’s something that often isn’t captured in a lab and why the ability to record real life at home is improving psychological research.

This story about tooth brushing  was written by Jill Barshay and produced by The Hechinger Report, a nonprofit, independent news organization focused on inequality and innovation in education. Sign up for the Hechinger newsletter.

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Jill Barshay writes the weekly “Proof Points” column about education research and data, covering a range of topics from early childhood to higher education. She taught algebra to ninth-graders for...

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