She added: "Parents should actively encourage their children to engage in a range of activities which promote their child's development and give them as much face-to-face time as possible".
"We were particularly interested in the long-term impact of screens, which is why we followed children over time, from ages 2 to 5, and repeatedly assessed both screen time and children's developmental outcomes", Madigan said.
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.
Higher levels of screen time at the ages of two and three years turned out to be "significantly associated" with poorer test results at three and five years.
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"Our study shows that preschool kids who get too much screen time, on video games, internet-connected devices, television screens, and other digital mediums are among those showing delays and deficits in learning by the time they enter school at the age of five".
Children in the study had an average of 17 hours of screen time a week when they were two years old, and 25 hours a week by the time they were three.
Madigan says that because children's brains are constantly developing over the course of childhood and beyond, there's still time to take steps in the right direction.
"It's creating some disparities in terms of children's development, where the kids who are watching screens are having some compromised development, whereas we're not seeing that with kids who have a low amount of screen time viewing", Madigan said in an interview. As a result, those children with poor language, coordination, as well as communication, problem-solving and social skills spent a lot of time in the front of a screen.
Nonetheless, he said, parents should balance screen time with other activities.
"Excessive screen time can impinge on children's ability to develop optimally", they write.
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Kalady said it's helpful to set up limits and expectations early, because it's easier to start off with healthy screen time habits than it is to scale back once you've already begun.
Because we only looked at total number of hours on screens, we don't know which apps, games or websites children are using.
She added that screens can also be used in a positive way, if the screen time is in moderation and when it's shared as a family. "And, as a generation, we're increasingly pressed for time".
The researchers reached that conclusion after analyzing and testing more than 2,400 children.
While allowing energetic or unruly children to watch videos or play with electronic devices may calm them down, too much screen time can impact their mental development.
Girls were more likely to score higher on developmental tests (β 0.23, 95% CI 0.18-0.27) and tended to have less screen time (β -0.06, 95% CI -0.11 to -0.02). Children benefit when parents devote more attention to them rather than relying on devices to occupy or calm them down.
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