Two Old Laws That Still Explain Most Interfaces
Fitts's law is about reaching a target. Hick's law is about choosing between options. Between them they account for a surprising share of what makes a screen easy or tiring.
Fitts's law: size and distance
Fitts's law is a predictive model of human movement. It says the time needed to move quickly to a target depends on the ratio between the distance to the target and the target's width. Far and small is slow. Near and large is fast. The summary and the mathematics are in the Wikipedia article on Fitts's law.
On a screen this explains several habits that feel like common sense once named:
- Primary buttons are large, and placed where the thumb or cursor already is.
- Destructive actions are small, or far from the confirm button, so that hitting them takes intent.
- Screen edges and corners are easy targets on a desktop, because the pointer stops there by itself.
Hick's law: the cost of choice
Hick's law, also called the Hick-Hyman law, describes the time it takes to make a decision as a function of the number of choices. Adding options increases decision time, though logarithmically and not in a straight line. See the Wikipedia article on Hick's law.
The practical reading is not "fewer options always" but "fewer options at once". Menus are grouped, forms are split into steps, and the most common choice is offered first.
Where the two meet
A checkout page with one large button and three clearly separated choices is fast for both reasons. A page with twelve similar-looking links of equal size is slow for both.
How the same tools get misused
Because these laws predict behaviour, they can be turned around. Make the option that benefits the business large and close, make the one that benefits the user small and distant, and most people will take the first without deciding to. That is the mechanism behind several of the practices described in the field guide to dark patterns.