The tendency to interpret new evidence as confirmation of one's existing beliefs or theories. Most people have their own specific beliefs on every aspect of life and it clouds them from seeing the real truth because they only see what they already believe in. This is why political campaigners focus on making their existing base show up for voting rather than trying to convince the other party because their base already believes what they have to say. As a marketer, you should not force too much to change someones mind, rather sell products/services that fuels their belief

Example: A person who believes that global warming is real may be more likely to believe news reports that confirm their beliefs, and less likely to believe reports that contradict their beliefs. This is an example of confirmation bias.