A group of former Facebook and Google employees last week began a campaign to change the tech companies they had a hand in creating. The initiative, called Truth About Tech, aims to push these companies to make their products less addictive for children — and it’s a good start.

But there’s more to the problem. If you think middle-class children are being harmed by too much screen time, just consider how much greater the damage is to minority and disadvantaged kids, who spend much more time in front of screens.

According to a 2011 study by researchers at Northwestern University, minority children watch 50 percent more TV than their white peers, and they use computers for up to one and a half hours longer each day. White children spend eight hours and 36 minutes looking at a screen every day, according to a survey by the Kaiser Family Foundation, while black and Hispanic children spend 13 hours.

While some parents in more dangerous neighborhoods understandably think that screen time is safer than playing outside, the deleterious effects of too much screen time are abundantly clear. Screen time has a negative effect on children’s ability to understand nonverbal emotional cues; it is linked to higher rates of mental illness, including depression; and it heightens the risk for obesity.

In 2004, Dimitri Christakis of Seattle Children’s Hospital wrote in the medical journal Pediatrics that “early exposure to television was associated with subsequent attentional problems.” Even when controlling for socioeconomic status, gestational age and other factors, he discovered that an increase of one standard deviation in the number of hours of television watched at age 1 “is associated with a 28 percent increase in the probability of having attentional problems at age 7.”

Every additional hour of TV increased a child’s odds of attention problems by about 10 percent. Kids who watched three hours a day were 30 percent more likely to have attention trouble than those who watched none. A 2010 article in Pediatrics confirmed that exposure to TV and video games was associated with greater attention problems in children. Meanwhile, Paul Morgan at Penn State and George Farkas at the University of California, Irvine, have found that black children are more likely to show symptoms of attention deficit hyperactivity disorder than their white peers.

But there is little evidence that such programs are helping students. Take Maine, which guarantees a tablet for every student. According to NPR, “at a cost of about $12 million annually,” the state “has yet to see any measurable increases on statewide standardized test scores.”

When politicians and policymakers talk about kids and technology, it is usually about “bridging the digital divide,” making sure that poor kids have as much access as wealthier ones. But there is no evidence that they don’t. According to a 2015 Pew report, 87 percent of Americans between the ages of 13 and 17 have access to a computer. For families earning less than $50,000 a year, that number is 80 percent. As for a racial divide, Pew finds that African-American teenagers are more likely to own a smartphone than any other group of teenagers in America.

These facts have not been allowed to get in the way of the shiny-new-things approach to learning. In 2014, New York received a half-million-dollar grant to lend internet hot spots to low-income families. According to the Urban Libraries Council, such lending programs are “the latest buzz.” Similar programs have begun in Chicago, Seattle and St. Paul, with funding coming from Google and other companies.

But no one is telling poorer parents about the dangers of screen time. For instance, according to a 2012 Pew survey, just 39 percent of parents with incomes of less than $30,000 a year say they are “very concerned” about this issue, compared with about six in 10 parents in higher-earning households.

Make no mistake: The real digital divide in this country is not between children who have access to the internet and those who don’t. It’s between children whose parents know that they have to restrict screen time and those whose parents have been sold a bill of goods by schools and politicians that more screens are a key to success. It’s time to let everyone in on the secret.