(Reuters) -Meta shut down internal research into the mental health effects of Facebook and Instagram after finding causal evidence that its products harmed users' mental health, according to unredacted filings in a class action by U.S. school districts against Meta and other social media platforms.

In a 2020 research project code-named "Project Mercury," Meta scientists worked with survey firm Nielsen to gauge the effect of "deactivating" Facebook and Instagram, according to Meta documents obtained via discovery. To the company's disappointment, "people who stopped using Facebook for a week reported lower feelings of depression, anxiety, loneliness and social comparison," internal documents said.

Rather than publishing those findings or pursuing additional research, the filing states, Meta called off further work and internally declared that the negative study findings were tainted by the "existing media narrative" around the company.

Privately, however, staff assured Nick Clegg, Meta's then-head of global public policy, that the conclusions of the research were valid.

"The Nielsen study does show causal impact on social comparison," (unhappy face emoji), an unnamed staff researcher allegedly wrote. Another staffer worried that keeping quiet about negative findings would be akin to the tobacco industry "doing research and knowing cigs were bad and then keeping that info to themselves."

Despite Meta's own work documenting a causal link between its products and negative mental health effects, the filing alleges, Meta told Congress that it had no ability to quantify whether its products were harmful to teenage girls.

In a statement Saturday, Meta spokesman Andy Stone said the study was stopped because its methodology was flawed and that it worked diligently to improve the safety of its products. 

"The full record will show that for over a decade, we have listened to parents, researched issues that matter most, and made real changes to protect teens," he said. 

PLAINTIFFS ALLEGE PRODUCT RISKS WERE HIDDEN

The allegation of Meta burying evidence of social media harms is just one of many in a late Friday filing by Motley Rice, a law firm suing Meta, Google, TikTok and Snapchat on behalf of school districts around the country. Broadly, the plaintiffs argue the companies have intentionally hidden the internally recognized risks of their products from users, parents and teachers.

TikTok, Google and Snapchat did not immediately respond to a request for comment. 

Allegations against Meta and its rivals include tacitly encouraging children below the age of 13 to use their platforms, failing to address child sexual abuse content and seeking to expand the use of social media products by teenagers while they were at school. The plaintiffs also allege that the platforms attempted to pay child-focused organizations to defend the safety of their products in public.

In one instance, TikTok sponsored the National PTA and then internally boasted about its ability to influence the child-focused organization. Per the filing, TikTok officials said the PTA would "do whatever we want going forward in the fall... (t)hey'll announce things publicly(,), (t)heir CEO will do press statements for us."

By and large, however, the allegations against the other social media platforms are less detailed than those against Meta. The internal documents cited by the plaintiffs allege:

1. Meta intentionally designed its youth safety features tobe ineffective and rarely used, and blocked testing of safetyfeatures that it feared might be harmful to growth. 2. Meta required users to be caught 17 times attempting totraffic people for sex before it would remove them from itsplatform, which a document described as "a very, very, very highstrike threshold." 3. Meta recognized that optimizing its products to increaseteen engagement resulted in serving them more harmful content,but did so anyway. 4. Meta stalled internal efforts to prevent child predatorsfrom contacting minors for years due to growth concerns, andpressured safety staff to circulate arguments justifying itsdecision not to act. 5. In a text message in 2021, Mark Zuckerberg said that hewouldn't say that child safety was his top concern "when I havea number of other areas I'm more focused on like building themetaverse." Zuckerberg also shot down or ignored requests byClegg to better fund child safety work.  Meta's Stone disputed these allegations, saying the company's teen safety measures are effective and that the company's current policy is to remove accounts as soon as they are flagged for sex trafficking. 

He said the suit misrepresents its efforts to build safety features for teens and parents, and called its safety work "broadly effective." 

"We strongly disagree with these allegations, which rely on cherry-picked quotes and misinformed opinions," Stone said.

The underlying Meta documents cited in the filing are not public, and Meta has filed a motion to strike the documents. Stone said the objection was to the over-broad nature of what plaintiffs are seeking to unseal, not unsealing in its entirety.

A hearing regarding the filing is set for January 26 in Northern California District Court.