Text size Videogame stocks slumped after a Chinese regulator said that young people in the country will be allowed to play online only three hours a week. The measure, announced Monday by the General Administration of Press and Publication, goes beyond an existing policy allowing 1½ hours of online play a day. It is aimed at curtailing what authorities in China have described as youth videogame addiction, likening it to substance abuse. Online games will be permitted from 8 p.m. to 9 p.m. on Friday, Saturday, and Sunday, as well as on public holidays. Shares of Roblox (ticker: RBLX) sagged...