Jurors in a landmark social media case that seeks to hold tech companies responsible for harms to children got their first glimpse into what will be a lengthy trial characterized by dueling narratives from the plaintiffs and the two remaining defendants, Meta and YouTube.

At the core of the Los Angeles case is a 20-year-old identified only by the initials "KGM," whose case could determine how thousands of similar lawsuits will play out. KGM and the cases of two other plaintiffs have been selected to be bellwether trials — essentially test cases for both sides to see how their arguments play out before a jury.

Comparing social media platforms to casinos and addictive drugs, lawyer Mark Lanier delivered opening statements Monday in the Los Angeles Superior Court trial that seeks to hold Instagram owner Meta and Google's YouTube responsible for addictive features and harms to children who use their products. Two other defendants, TikTok and Snap, have settled the case.

Meta lawyer Paul Schmidt spoke of the disagreement within the scientific community over social media addiction, with some researchers believing it doesn't exist, or that addiction is not the most appropriate way to describe heavy social media use.

Luis Li, the attorney representing YouTube and Google, delivered an opening statement on Tuesday focused on KGM's user data, saying the five-year average of her watch time is 29 minutes per day. He said KGM's average daily time spent on YouTube Shorts, watching vertical short form videos with the "infinite scroll" feature Lanier called into question Monday, was just 1 minute and 14 seconds.

He also told jurors that all of the YouTube features Lanier challenged in his opening statement could be disabled and modified to match users' preferences.

Lanier, the plaintiff's lawyer, delivered lively first remarks Monday, where he said the case will be as "easy as ABC" — which stands for "addicting the brains of children." He said Meta and Google, "two of the richest corporations in history," have "engineered addiction in children's brains."

He presented jurors with a slew of internal emails, documents and studies conducted by Meta and YouTube, as well as YouTube's parent company, Google. He emphasized the findings of a study Meta conducted called "Project Myst," in which they surveyed 1,000 teens and their parents about their social media use. The two major findings, Lanier said, were that Meta knew children who experienced "adverse events" like trauma and stress were particularly vulnerable for addiction; and that parental supervision and controls made little impact.

He also highlighted internal Google documents that likened some company products to a casino, and internal communication between Meta employees in which one person said Instagram is "like a drug" and they are "basically pushers."

Li was insistent that KGM is not addicted to YouTube, pointing to sworn testimony where she directly said she wasn't addicted to it. He also displayed three large boxes containing about 10,000 pages of medical records, saying that within all of those records, jurors would not see a "single example" of KGM being addicted to YouTube.

The sole reference to YouTube within those records is an instance where her provider noted KGM was using a YouTube video to assist with sleep at night when feeling anxious, he said.

KGM made a brief appearance on Monday during Lanier's statement and she will return to testify later in the trial. Lanier spent time describing KGM's childhood, focusing particularly on what her personality was like before she began using social media. She started using YouTube at age 6 and Instagram at age 9, Lanier said. Before she graduated from elementary school, she had posted 284 videos on YouTube.

The outcome of the trial could have profound effects on the companies' businesses and how they will handle children using their platforms.

Lanier said the companies' lawyers will "try to blame the little girl and her parents for the trap they built," referencing the plaintiff. She was a minor when she said she became addicted to social media, which she claims had a detrimental impact on her mental health.

The attorney also drew comparisons between the social media companies and tobacco firms, citing internal communication between Meta employees who were concerned about the company's lack of proactive action about the potential harm their platforms can have on children and teens.

In his opening statement representing Meta, Schmidt said the core question in the case is whether the platforms were a substantial factor in KGM's mental health struggles. He spent much of his time going through the plaintiff's health records, emphasizing that she had experienced many difficult circumstances in her childhood, including emotional abuse, body image issues and bullying.

Schmidt presented a clip from a video deposition from one of KGM's mental health providers, Dr. Thomas Suberman, who said social media was "not the through-line of what I recall being her main issues," adding that her struggles seemed to largely stem from interpersonal conflicts and relationships. He painted a picture — with KGM's own text messages and testimony pointing to a volatile home life — of a particularly troubled relationship with her mother.

Schmidt acknowledged that many mental health professionals do believe social media addiction can exist, but said three of KGM's providers — all of whom believe in the form of addiction — have never diagnosed her with it, or treated her for it.

A slew of trials beginning this year seek to hold social media companies responsible for harming children's mental well-being. Executives, including Meta CEO Mark Zuckerberg, are expected to testify at the Los Angeles trial, which will last six to eight weeks.

A separate trial in New Mexico, meanwhile, also kicked off with opening statements on Monday. In that trial, Meta is accused of failing to protect young users from sexual exploitation, following an undercover online investigation.

A federal bellwether trial beginning in June in Oakland, California, will be the first to represent school districts that have sued social media platforms over harms to children.

In addition, more than 40 state attorneys general have filed lawsuits against Meta, claiming it is harming young people and contributing to the youth mental health crisis by deliberately designing features on Instagram and Facebook that addict children to its platforms. The majority of cases filed their lawsuits in federal court, but some sued in their respective states.