What do the words “dragon,” “baseball” and “pussy” have in common? They all regularly appear on lists of the most commonly used computer passwords. They make the grade year after year because dragons, baseball and pussies (meaning cats, of course) are all things that people like. Human sentimentality is the enemy of internet security, which makes banks’ obsession with knowing our first pets’ names and the streets we grew up on seem counterintuitive; these are words that stick in our mind, but they are not reliable gatekeepers.

In a study of the psychology of password selection, Helen Pet-rie, professor of human-computer interaction at the University of York, undertook a survey of 1,200 British computer users and established four main password personality classifications...