People Interface Design
Via Mike Davidson and Jeffrey Veen:
To predict the behavior of ordinary people in advance, you only have to assume that they will always try to escape a disagreeable situation with the smallest possible expenditure of intelligence. - Friedrich Nietzsche
To quote Jeffrey:
This helps explain simple anomalies in user behavior at your site. Why do people search for terms that are listed right in the navigation? Because it's easier to type the word you want than pick it out of a list.