Game developers have always been interested in how players might react to the characters and plots they created—but what if they could tell exactly how the player was feeling and tailor the game to their mood?

“Back in the olden days we had to do a lot of guesswork as game designers,” says Erin Reynolds, the creative director of the gaming company Flying Mollusk. “Is the player enjoying this? Is the player bored? You had to create a game that was one size fits all.”