The effects of Portal 2 and Lumosity on cognitive and noncognitive skills
Valve playtests the shit out of their games. A lot of Portal's theming is based on their extensive testing culture and the ways they've developed to manipulate the player through testing. For example, the Weighted Companion Cube came about because they wanted a level where you solve puzzles while carrying an object. But the playtesters kept leaving the damn object behind. So Valve's solution was to tell them it was a "character" they had to care for.