Why they lie to you:

Because of tricks the brain plays, Mr. Miller wrote, “we’re likely to believe anything we hear repeated frequently enough. At FactCheck.org we’ve noted how political spin-masters exploit this tendency ruthlessly, repeating dubious or false claims endlessly until, in the minds of many voters, they become true. Making matters worse, a study by Hebrew University’s [Ruth] Mayo shows that people often forget ‘denial tags.’ Thus many people who hear the phrase ‘Iraq does not possess WMDs’ will remember ‘Iraq’ and ‘possess WMDs’ while forgetting the ‘does not’ part.”

Good to know it’s our fault.

A similar study, or at least conclusion, was discussed over at Ars Techinca:

Volunteers were shown news items or political adverts that contained misinformation, followed by a correction. For example, a study by John Bullock of Yale showed volunteers a political ad created by NARAL that linked Justice John Roberts to a violent anti-abortion group, followed by news that the ad had been withdrawn. Interestingly, Democratic participants had a worse opinion of Roberts after being shown the ad, even after they were told it was false.

One has to appreciate how stupid we really are.