If a friend tells you that a fuzzy picture of a mango is actually a banana (and you switch your opinion to believe him/her), the next day you will be more entrenched in your belief in the picture’s banana-ness than you would be in its mango-ness had your friend simply confirmed the truth. It’s as if −1 + 1 is greater than 1 + 1.
Cool, eh?
This is true to the point that even when the picture is made less fuzzy, gaining obvious mango-ness, you’ll continue to see and believe banana if you’ve had time to solidify your trusted friend’s belief—your entrenched wrong opinion resists the truth much more than a confirmed correct opinion.
It’s as if believing a lie requires more dogmatic determination and thus holds tighter than the easy belief in the truth.
There are parallels to be made in human society.