Social media companies will have to answer to a jury – for the first time – for allegations that their products are intentionally addictive and harmful to young users’ mental health. Hundreds of parents, teens and school districts sued Meta, Snap, TikTok and YouTube, leading to a series of landmark trials that began this week. Jury selection in the first case started on Tuesday in Los Angeles court.

Meta’s Mark Zuckerberg is among the big tech CEOs who are expected to testify. Both sides are likely to bring in experts to hash out the science behind alleged addiction to social media.

The initial trial involves a 20-year-old, identified by the initials KGM, who alleges she experienced physical and emotional harm after becoming addicted to social media at age 10. This case is expected to last six to eight weeks, and will be the first of about 22 “bellwether” trials. The outcome can signal, and influence, how courts and juries may handle similar lawsuits in the future. Snap and TikTok have settled with the plaintiff in the first case, leaving Meta and YouTube to stand trial.

“Everybody’s looking at it as a data point,” said Benjamin Zipursky, a law professor at Fordham University.

The legal proceedings follow years of heated criticism of tech companies from angry parents; they blame social networks for causing eating disorders, suicidal thoughts and anxiety in their children. The companies lay blame on the victims, plaintiffs’ lawyers and supporters say: “The tech companies blame parents for not being good parents,” said Sacha Haworth, executive director of the Tech Oversight Project, which has been involved in advocacy work for the plaintiffs. “Their experience has shown that you can be the best parent in the world – you can have conversations, you can limit screen time, you can turn on the bullshit parental controls – and your kid still dies.”

Why are these cases unprecedented?

The cases and future settlements could eventually change the way social media platforms work; plaintiffs are asking for money, as well as injunctive relief that would establish industry-wide safety standards. Legislators around the world are watching; many are trying to impose liability on tech companies in a similar way as the plaintiffs. So even if juries declare the defendants aren’t liable in these cases, lawmakers will be taking notes–and adapting their own strategies, says Eric Goldman, a professor at the Santa Clara University school of law. Some have already enacted laws that align with the plaintiff’s claims.

There’s more at stake now, legal experts say. “Bad press about how some members of Congress embarrass Zuckerberg is one thing,” Zipursky added. Changing the architecture of a social network may be another entirely.

What are the legal arguments?

Plaintiffs want to know: how much did tech companies know about the alleged harm caused by their products, and to what extent did they fail to warn users? They take particular issue with design features such as infinite scroll, autoplay and content curated by algorithms, accusing social media companies of prioritising engagement over wellbeing. Their legal strategy has brought up comparisons with those used against tobacco companies in the 1990s.

The trial will explore what social media addiction consists of. Tech companies have questioned the scientific link between their platforms and addiction. They also highlight various safety features and parental controls they adopted in recent years. Even if this harm exists, they argue, it’s a result of problematic content, as opposed to a poorly designed system. In doing so, the companies have often relied on a federal law that shields them from liability for third-party content on their platforms: section 230 of the Communications Decency Act. That defense hasn’t worked so far in this case, given the plaintiffs’ novel argument.

“[Plaintiffs] have reconfigured it to say, look: the wrongful thing Meta is doing is running its whole platform in such a way that people get addicted,” Zipursky said. A key issue will be whether plaintiffs can prove the tech companies were wrongfully taking harmful risks, Zipursky added. Moreover, if the plaintiffs can prove that tech companies knowingly and intentionally caused harm, it will be easier to get punitive damages.

Social media addiction is not included in the latest edition of the Diagnostic and Statistical Manual of Mental Disorders, the authoritative handbook for mental health treatment. Scientists have, however, catalogued compulsive use with severe consequences among young people, and legislators across the world are engaged in heated debate over its addictive potential and how to respond.

Which tech executives are testifying?

Meta CEO Mark Zuckerberg, YouTube CEO Neal Mohan and Instagram CEO Adam Mosseri are expected to testify in the initial trial. Snap and TikTok reached a settlement in that case but are still defendants in other suits in the state-level trial.

A Meta spokesperson said in an emailed comment that they strongly disagree with these allegations, and stressed that they have listened to parents and worked with experts and law enforcement to address safety issues. The company also cited the introduction of teen accounts, with additional protections.

Google spokesperson José Castañeda called allegations in the lawsuits “simply not true”, and emphasized that the company had prioritized providing a safe and healthy experience for young people. Snap commented only on the first bellwether trial, saying they were “pleased to have been able to resolve this matter in an amicable manner”. TikTok did not immediately respond to a request for comment.

What evidence is already unsealed?

While much of the cases is yet to play out, some unsealed evidence has already drawn criticism of tech companies. That includes allegations that an Instagram employee compared the app to a drug, and that another suggested: “lol, we’re basically pushers.” A Meta spokesperson previously told Politico that the plaintiffs’ allegations relied on “cherry-picked quotes and misinformed opinions in an attempt to present a deliberately misleading picture”.

The 20-year-old at the center of the initial trial started using social media around age 10, even as her mother tried to stop her from using the apps. She “developed a compulsion to engage with those products nonstop” as a result of their “addictive design” and “constant notifications”, the complaint notes, according to NPR.

Will big tech win?

Legal experts expect all of these cases will eventually be appealed if they don’t reach a settlement, and there’s no certainty about outcomes until appellate courts have weighed in. They expect tech companies to raise several issues, including whether the Communications Decency Act or first amendment applies, as well as potential disputes over causation and expert testimony. “None of these companies are going to feel that they need to change what they do,” Zipursky said.

Still, juries’ verdicts in the early cases can affect how much companies may pay out. “The jury might signal they’re completely persuaded by the plaintiff’s arguments,” Goldman said. “Then, if there’s any settlement, the price tag will go up substantially.”