The findings, published in the Journal of Developmental & Behavioral Pediatrics, reinforce the need for limits on screen time, such as those laid out by the American Academy of Pediatrics (AAP).

In its 2001 guidelines, the AAP recommended that children over the age of 2 watch no more than two hours of television per day. These guidelines, updated in October 2016, now recommend that children between 2 and 5 watch no more than one hour of television.

"Given that studies have reported that children often watch more than the recommended amount, and the current prevalence of technology such as smartphones and tablets, engaging in screen time may be more frequent now than ever before," said Andrew Ribner, a doctoral candidate in the Department of Applied Psychology at NYU Steinhardt and the study's lead author.

Research has shown that watching television is negatively associated with early academic skills, but little is known about how socioeconomic status influences television viewing and child development. In the current study, the researchers examined whether the negative relationship between watching television and school readiness varied by family income.

Ribner and his colleagues looked at data from 807 kindergartners of diverse backgrounds. Their parents reported family income, as well as the number of hours of television their children watch on a daily basis. Video game, tablet, and smartphone use were not included in the measurement.

Children were assessed using measures of math, knowledge of letters and words, and executive function -- key cognitive and social-emotional competencies, including working memory, cognitive flexibility, and inhibitory control, that are viewed as fundamental for school readiness.

The researchers found that the number of hours of television young children watch is related to decreases in their school readiness, particularly their math skills and executive function. This association was strongest when children watched more than two hours of television.

As family incomes decreased, the link between television watching and drops in school readiness grew, meaning children from low-income families are hurt more by watching too much television. Those at or near the poverty line (an annual income of around $21,200 for a family of four) saw the largest drop in school readiness when children watched more than two hours of television. A more modest drop was observed among middle-income families (measured as $74,200 per year for a family of four), while there was no link between school readiness and television viewing in high-income homes (measured as around $127,000 per year for a family of four).

Interestingly, while television viewing was negatively associated with math skills and executive function, a similar link was not found with letter and word knowledge. The researchers speculate that television programming, especially educational programs for children, may work to improve literacy among young children in ways that are not found in math.

While the study did not measure the type of content the children watched, nor the context of their television viewing, the researchers note that both may be relevant to their findings, particularly in understanding why more affluent families appeared to be protected from the decline in school readiness linked to too much television.

For instance, children in higher-income homes may be watching more educational programming and less entertainment, which has been found in earlier studies. In addition, more affluent parents may be more likely to watch television with their children -- offering explanation and discussion that can promote understanding -- based on having more time and resources.

"Our results suggest that the circumstances that surround child screen time can influence its detrimental effects on learning outcomes," said Caroline Fitzpatrick of Canada's Université Sainte-Anne, who is also an affiliate researcher at Concordia University and a coauthor on the study.

The researchers recommend that efforts be made by pediatricians and child care centers to reinforce the AAP guidelines and help parents limit the amount of television children watch to fewer than two hours a day.

In addition to Ribner and Fitzpatrick, Clancy Blair, professor of cognitive psychology at NYU Steinhardt, coauthored the study.

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