In 1912 in the village of Piltdown, on the south coast of England, Charles Dawson discovered a skull in the local gravel beds. 

The skull, later named Piltdown Man, revolutionized archeology as the "missing link" between apes and man. Little did the world know this was the start of a great deception.

It took 40 years for the truth to emerge: Piltdown Man was a hoax. The "missing link" was an orangutan's jaw bone combined with a portion of a modern man's head. The teeth had been filed down, cracks filled with dentist putty, and the bones stained to match local rocks.

Competent paleontologists at the time should have dismissed this "discovery" outright. But they didn't. Why?

Confirmation bias—a tendency to favor information that confirms existing beliefs and hypotheses—was at work.

1) Dawson tapped into a competitive spirit.

With an earlier discovery of an ancient jawbone in 1907 near Heidelberg, Britain was looking for a way to reclaim its place at the forefront of science. The discovery "fit."

Principle: Confirmation bias is strongest for desired outcomes, emotionally charged issues, and deeply entrenched beliefs.

Lesson: When "everyone agrees," look more closely, curiously, and skeptically.

2) Charles Dawson showed people what they wanted to see.

Long before, scientists predicted a missing link between apes and man, and they were waiting to find the evidence. 

Principle: We search for, favor, and recall information that supports our beliefs—even interpreting ambiguous information as supporting the view and ignoring contrary information or evidence. 

Lesson: If your information is limited, extend your research, ask more questions, and head out "into the field" to look for yourself.