Hick's Law
Hick's Law is a principle of design that states that the time for a human to take a decision is based on the number of possibilities it has, and increases logarithmically. It is part of a study on Computer-Human interaction, where we study human patterns and actions to have a better understanding of the end user when building software. The importance of the field is rising nowadays, with software developers having in mind the importance of delivering the software the user needs, and understanding what he needs is often really hard.
Hick's Law has some exceptions and can be studied on differente IQ's and stimulus to differente results, but for the most part, it is a important principle of design.