How do games stay hard enough to thrill but easy enough to finish?

A game has to be hard enough to be exciting and easy enough to be finished, and the right amount is different for every player. Designers often borrow an idea from psychology to describe the target. Flow, the absorbed state people call being in the zone, needs a good balance between the challenge of a task and your skill. If the challenges are few and your skill exceeds them, you get bored. If the challenges exceed your skill, you get anxious.

One way to hit the target is to let the game itself notice how you are doing and adjust. This is called dynamic difficulty balancing. The game uses some measure of how hard it feels to you at the moment, and nudges its settings to keep you between boredom and anxiety.

The nudges can be tiny. In the 1975 arcade game Gun Fight, the player who had just been shot got a cactus on their half of the screen, so it was easier to hide. In the first Crash Bandicoot, dying more often could slow obstacles, give extra hit points and add continue points. Its lead designer said the aim was to help weaker players without changing the game for better ones. Resident Evil 4 went further: it graded your performance on a scale from 1 to 10 and adjusted its enemies to match, and the only mention of the system was in the official strategy guide.

The secrecy is deliberate. When players spotted that one game made its levels easier or harder depending on how they had just played, some learned to play badly on purpose before the hard levels. That is why the designers Andrew Rollings and Ernest Adams stress covering up difficulty adaptation, so that players never notice it.