We all have a tendency to pay more attention to evidence that supports what we already believe or want to be true and to ignore, neglect or dismiss evidence that contradicts of favoured beliefs.
In one study, participants were presented with information relating to the JFK assassination. Some of the information was more consistent with the lone gunman explanation, some was more consistent with the conspiracy-based explanation. All participants found the information that was consistent with their pre-existing views more convincing than information that was inconsistent with those views. Thus the presentation of the same information had a polarising effect: those who already believed in the conspiracy theory believed in it even more strongly; those who already rejected the conspiracy theory, rejected it even more strongly.
…from an article in The Guardian