In two landmark cases, social media companies have been found liable for endangering and harming children. Meta and Google are appealing the verdicts and disputing the idea that their products are addictive. But over the course of more than a decade, scientists have identified key features of social media and other apps meant to hold children's attention for as long as possible.

These features create a kind of superglue on the apps, says cultural anthropologist Natasha Dow Schüll at New York University, who has pioneered research in this field. "They keep us spending more time on these apps and spending more money. They drain us of our energy and ourselves." Understanding these features offers parents a rubric for evaluating how harmful an app or device may be for kids, Schüll says.

During the trial in California, the attorney bringing the case accused Meta and Google of designing their apps to behave like "digital casinos." That's an apt comparison, according to Schüll's research, because major design elements of social media have surprising roots in the gambling industry.

Pulled into the "machine zone"

Back in the 1980s and 1990s, the casino industry gradually and purposely created what many scientists consider to be the most addictive form of gambling: video slot machines. They are something like a giant app, played on a huge video screen with an ergonomic chair attached to it.

People struggling with gambling addiction often cite video slots as their game of choice, studies have found. Some people gamble on these machines for extraordinary periods of time, Schüll found in her ethnographic fieldwork. They can play for 24 hours, even 48 hours straight. Some people even told Schüll that they wear adult diapers to the casino so they don't have to stop gambling to use the restroom.

Thirty years ago, Schüll set out on a bold mission: to figure out how these games exert this magnetic effect. What features might literally prevent flourishing?

She spent 15 years dissecting the inner workings of video slot machines. She also interviewed everyone up and down the industry, from the marketers and mathematicians to software engineers and executives, as well as people who used these devices daily.

Through her research, she uncovered four key features that, when combined together, help hold people on the gambling devices. These features trigger a trancelike or dissociative state, known as a "machine zone" or "dark flow," in which people lose track of their sense of time and place.

To Schüll's surprise, around the early 2010s, the same features began to appear on phone and tablet apps, including social media, games and video-streaming platforms. "These are not normal products for kids like a pair of shoes or a toy," she says. "They create a relationship with kids."

Here are four features that create that superglue:

Feature 1: solitude

"When the relationship is just between you and the machine, it removes social cues needed for stopping," Schüll says. It's harder to notice when the activity no longer serves the person playing or scrolling.

Studies have found that children who regularly use screens alone in their bedrooms have a higher risk of developing what psychologists call problematic usage. That is, they continue to use an app or play a game even when it damages their health. For example, the app may interfere with their sleep or friendships, but the child still feels compelled to stay on the app.

Feature 2: bottomlessness

Videos keep appearing on TikTok and YouTube. Photos, comments and likes keep popping up on Instagram. Apps have seemingly endless content for you to see, and it all shows or plays automatically.

"There's no natural stopping point," Schüll says. So you never feel finished or satisfied.

You want one more of something, endlessly. And that feeling grows even stronger with the third ingredient added into the mix.

Feature 3: speed

The faster people play video slots, the longer people gamble, Schüll found in her review of research performed by the gambling industry. Speed has a similar effect on social media and video-streaming apps, she says. The faster people can scroll, watch and then watch again, the harder it is for many to pull away from an app.

"The speed of the feedback can cause this sense that you merge with the screen. You don't know where you begin and the machine ends," Schüll says. "The speed really just pulls you into this flow."

For social media, the speed at which we can find "new" material has jumped with several technological advancements, including the invention of higher-speed internet and infinite scroll.

Feature 4: teasing, or giving you almost what you want

The final ingredient is perhaps the most important, says Jonathan D. Morrow, a neuroscientist and psychiatrist at the University of Michigan. It's all about how apps select content for you.

Here's how it typically works. First, the software uses AI to determine what you're hoping to find or see. "Even if you don't know what you want, the app knows. It's very good at figuring that out," Morrow says.

But then, he says, the app withholds that reward: "Apps don't give it to you. They give you something close to that, and then a few clicks later, the algorithm gives you something even closer."

They rarely — if ever — give you what you're looking for. "They give just enough to keep you engaged, keep you looking at the app and interacting with it as long as possible," he adds.

This teasing gives you the feeling that you're going to get what you're seeking soon. "So you'll be there all day trying to get that next big thing. There's always a possibility you'll finally get what you want," Morrow says.

A recipe for overuse

When an app combines these four features — solitude, bottomlessness, speed and teasing — it creates a kind of recipe for overuse for nearly everyone, Schüll says. Sometimes Schüll gives her students at New York University this list of design features. "I say, 'Pick a website or app. Then, using these criteria, rate how harmful it is.'"

But the recipe is especially harmful for children, she adds: "It's a cruel setup, especially when kids are concerned. Kids are obviously more vulnerable." Therefore, she and Morrow agree: Children need help regulating their use of these apps, but they also need protection from harmful design.

Michaeleen Doucleff has a Ph.D. in chemistry and is a longtime science journalist (including previously for NPR). She is the author of the parenting book Dopamine Kids.