(CNN) -- Among toddlers, spending a lot of time staring at screens is linked with poorer performance on developmental screening tests later in childhood, according to a new study.
The study, published in the journal JAMA Pediatrics on Monday, found a direct association between screen time at ages 2 and 3 and development at 3 and 5.
Development includes growth in communication, motor skills, problem-solving and personal social skills, based on a screening tool called the Ages and Stages Questionnaire. Signs of such development can be seen in behaviors like being able to stack a small block or toy on top of another one.
The American Academy of Pediatrics recommends limits on screen use for preschool children ages 2 to 5 to just one hour a day of high-quality programming.
"On average, the children in our study were viewing screens two to three hours per day. This means that the majority of the children in our sample are exceeding the pediatric guidelines of no more than one hour of high-quality programming per day," said Sheri Madigan, an assistant professor and research chair in determinants of child development at the University of Calgary, who was first author of the study.
"Higher screen time viewing at 2 and 3 years of age was associated with children's delays in meeting developmental milestones at 3 and 5 years of age, respectively," she said. "This study shows that, when used in excess, screen time can have consequences for children's development. Parents can think of screens like they do giving junk food to their kids: In small doses, it's OK, but in excess, it has consequences."
For older children, parents are encouraged to develop personalized media plans for their children, but the American Academy of Pediatrics notes that all children and teens need at least around eight hours of sleep, one hour of physical activity and time away from media each day.
A separate report released in 2017 by the nonprofit organization Common Sense Media found that children 8 and younger spend an average of two hours and 19 minutes a day with screen media.
Most children of all ages in the United States spend a total of about five to seven hours a day in front of a screen, including watching TV, working on a computer or playing video games, according to the US National Library of Medicine.
The new study included data on 2,441 mothers and children in Canada. The mothers were recruited for the study when they were pregnant between 2008 and 2010, and data was collected between 2011 and 2016.
For each child in the study, the mother completed questionnaires related to the child's performance on developmental tests at ages 24, 36 and 60 months. Mothers also reported the range of time their children spent using screen devices on a typical weekday and weekend day.
The researchers found that greater screen time at 24 months was associated with poorer performance on developmental screening tests at 36 months, and greater screen time at 36 months was associated with lower scores on developmental screening tests at 60 months.
Although the researchers did not examine the relationship between screen time and developmental outcomes numerically, they found "a stable association" between screen time and child developmental screening test scores that was not accounted for by other factors, according to the study.
The average amount of screen time for children ages 24, 36 and 60 months in the study was about 2.4, 3.6 and 1.6 hours per day, respectively, the researchers found.
"To our knowledge, the present study is the first to provide evidence of a directional association between screen time and poor performance on development screening tests among very young children," the researchers wrote.
The study had some limitations, including that some of the data was self-reported by the mothers and that devices can rapidly evolve over time, which could influence screen time.
Also, the first assessments of the children in the study were at 24 months, whereas assessments starting at 12 or 18 months could have provided more data.
Most important, just because an association between excessive screen time and poorer child development was found doesn't mean excessive screen time causes poorer development.
"Our study identifies a correlation between two things, and this does not mean that one causes the other," Madigan said.
The new study provides insight into how technology impacts individual young people over time, "but increases in screen time observed here indicate between about 0.36% 0.64% of the variability in decreases in the developmental outcome. This means that upwards of 99% of the children's developmental trajectories studied here have nothing to do with screens," Andrew Przybylski, an associate professor and director of research at the University of Oxford's Oxford Internet Institute, said a written statement released by the independent Science Media Centre on Monday.
"The conclusions drawn are overly strong for the method used," said Przybylski, who was not involved in the study.
"Much of this study is well conducted, which is good in a research field where many studies are poorly done but there are huge limitations to be aware of in terms of the practical implications of the work," he said, adding that it is premature to advise that "limiting screen time alone will improve developmental outcomes for children in any meaningful way."
Some important factors could help explain the links between higher levels of screen time and poorer performance on developmental screening tests, said Douglas Gentile, a psychology professor at Iowa State University who was not involved in the new research but has studied the effects of media use on children.
"It is notable that screen time reduced both children's sleep even at this early age and reduced parents' reading to children, which we know is a strong predictor of positive child outcomes, such as higher IQ," said Gentile, who called the new study "strong" and "well-conducted."
The American Academy of Pediatrics offers guidelines to help families with the management of children's screen time. Those guidelines include avoiding digital media for toddlers younger than 18 to 24 months, except for video chatting.
Overall, "the good news is that screen time is something parents can control," Gentile said. "In other studies, we've found that when parents put limits on the amount and content of children's screen media, it is a powerful protective factor for a wide range of children's health and wellness indicators."
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