OAKLAND, Aug. 21 -- Among the thousands of pages of internal research that Meta kept from shareholders, regulators, and the public for years, one figure stood out: one in four teenage girls who used Instagram reported feeling worse about their bodies after spending time on the platform. That research existed. It was suppressed. And it was Arturo Bejar who found it, who on Thursday told a federal jury in Oakland how that suppression happened, and who made it happen.

Bejar, a senior engineering director at Meta from 2009 to 2015 who later returned as an independent contractor examining the well-being of teenage Instagram users, testified that CEO Mark Zuckerberg built a corporate culture where child safety occupied a structurally secondary...