Why does L sit in a crouch?

The in manga answer is a direct quote, delivered deadpan to the task force: "I just can't sit any other way than this. If I sit the way other people do, my reasoning ability drops by 40%." The number is not measured, sourced, or explained, and the series never revisits it, which is rather the point. L states his own operating conditions the way other detectives state facts, and the investigation rearranges the chairs. The crouch, knees up, feet flat on the seat, thumb at his lip, is presented as equipment: less a quirk than a workstation his body built.

The creator side of the answer is the finest detail in the whole interview record. Tsugumi Ohba, the writer who invented that posture, develops manga plots while holding their own knees on a chair. The habit was documented in Ohba's profiles years into the run: the anonymous writer thinks in the position of their most famous thinker. Whether the crouch began as the writer's habit lent to the character or converged later, the record shows the two sharing it, the creator folded into the detective's shape at a desk nobody has ever seen.

Read the pose with that in mind and its meaning sharpens. Obata draws the crouch as the opposite of Light's posture: the perfect student sits straight, speaks carefully, and performs normality, while the detective who sees through him cannot sit like a person at all. The forty percent is a joke with a thesis inside it. L's genius is inseparable from his refusal to be arranged, and the chair is where the refusal starts.