When Valve first got its hands on Turtle Rock's Counter-Strike mod, Left 4 Dead, it tried upping the number of players from four to five or six, even after Turtle Rock said it wouldn't turn out well. Valve tried it and it didn't work out, and Left 4 Dead shipped with four playable characters.

Evolve, Turtle Rock's next-gen monster shooter, sticks with four hunters fighting against one monster (hunters shown above). Turtle Rock isn't sure why four players works out best, but creative director Chris Ashton shared his theory with Game Informer:

"What happens is there's a weird thing in that most people I think are able to track three friends. I can know that you're over here and you're in front of me and you're to my left. And I can keep that in my mind, and I can keep in my mind that you have 50 health and you have 80 health, and I can keep track of that and fight another team. But if it's four guys, it feels like I'm always losing one. I always don't know where someone is, I don't know where somebody's health is – keeping track of four other friends is too much."

Those who play as the super-powered monster will have their hands full keeping tabs on all four enemies, Ashton said. "That's what makes it a challenge for him," he said. "As soon as you kill one guy and get one guy out of the picture, I think three humans are way easier to deal with and keep mental tabs on."

Evolve is due out this fall for Xbox One, PS4 and PC. And speaking of Left 4 Dead – is that Bill on the far left in that image up there? Or the far right? Or do we just really miss Bill?

This article was originally published on Joystiq.