When faced with mixed evidence, people focus on the parts that confirm their beliefs and ignore the parts that don't. A believer in birth order who didn't happen to know the birth order of Bill Gates, the founder of Microsoft, might think he was a firstborn, because of his leadership ability and drive to succeed. A believer in birth order who happens to know that Bill Gates is the second of three siblings will focus on some other aspect of his personality, such as his openness to new ideas or his loyalty to old friends.

If the disconfirmatory evidence can't be ignored, it can be explained away. Kevin Leman, who accurately describes himself as "North America's 'pop' birth order psychologist" (1998, p. 13), believes that firstborns are meticulous, well-organized perfectionists, but was undismayed when he encountered a firstborn with a messy desk. "Your desk is sloppy," he observed, "but can you find what you need on it?" "Of course," was the firstborn's reply. Satisfied, Leman went on to speculate: "My guess is you are something of a perfectionist and perfectionists are known for having sloppy desks as a means of covering their discouragement for not always having life go just the way they want it" (1998, p. 93). It's a win-win setup for Leman: If a firstborn has a neat desk, it's because he's a perfectionist, and that confirms Leman's theory. If a firstborn doesn't have a neat desk, it's because he's covering up his discouragement, and Leman's theory is again confirmed.