Meta and Google, two of the most powerful companies in history, have long rejected claims their social media platforms are inherently dangerous for kids. On Monday, they are beginning a landmark trial over allegations that Meta’s Instagram and Google’s YouTube were intentionally designed to get young users hopelessly hooked despite known perils.

The so-called “bellwether” case is the first to reach trial among thousands of similar lawsuits from individual plaintiffs, school districts, and state officials. It was filed in July 2023 by a now 20-year-old California woman, identified by the initials K.G.M., who says she became desperately addicted to social media as a child, got flooded with disturbing material she did not seek, and subsequently experienced body dysmorphia, anxiety, self-harm, and suicidal thoughts.