Hick's Law states that the time it takes to make a decision increases with the number of alternatives.
For example, when you say to your partner, "What do you want for dinner tonight?" there's a really good chance -- especially if this is a nightly conversation -- that your partner's brain will stall going through all of the options. On the other hand, if you say "What do you want for dinner tonight, chicken or fish?" there's a really good chance they will make a decision much more quickly because there are only two options.
In Designing for Emotion, Aaron Walter suggests that Tumblr's "dead-simple homepage" -- consisting of only a registration form -- uses Hick's Law as a design heuristic. By removing as much noise and as many barriers to registration as possible, the designers have acted as a filter to noise that otherwise the user's brain would have to filter.
Tumblr recognizes that attention is a finite commodity. Every time we add content to an interface, it makes it harder for humans to identify patterns and contrasting elements. The result is more unpredictable user behavior, and lower information retention. (Remind your boss fo that the next time you're asked to shoehorn more stuff into your company's homepage.
Aaron Walter, Designing for Emotion
- Hick's Law on Laws of UX