How can people believe something that isn’t true?

We've been talking a lot about 9/11 in class and have discussed various conspiracy theories and the publics perception of 9/11 & the Iraq War.
Looking through Google News I found this interesting article from CBS News (9/12/07):

Polls, Truth Sometimes At Odds
CBS' Kathy Frankovic: Political Views, Psychology Can Make People Believe In Falsehoods

How can people believe something that isn’t true?

A significant number of Americans say Saddam Hussein was personally involved in the September 11, 2001 terrorist attacks on the World Trade Center and the Pentagon. That number rose above 50 percent in the run-up to the war in Iraq in 2003. While that belief has since declined somewhat, for many Americans it still exists. In the latest CBS News/New York Times Poll 33 percent said they believe Saddam was personally involved in the 9/11 attacks. As has been repeated over and over, there was and is no evidence of any such link. So why do so many people believe something that just isn’t true?

What do you think?
You can read the rest of the article here.