ihadnoidea: I think to a degree we all are affected by confirmation bias.
More than just to a degree. Confirmation bias is just one of the ways our brain works, in order to minimize the amount of time and energy devoted to anything aside from being aware of our surroundings. Those who could compartmentalize and quickly process decision-making info were less likely to be caught unawares by a predator.
Our brains process a lot of information automatically and manage our vitals with minimal or no input. Even when we're aware of behavior like confirmation bias, we're still subject to it because it's how our brains work. A good pair of books that review research on how our brains work would be the ones by David McRaney: You Are Not So Smart and You Are Now Less Dumb. Some of the research is limited and probably out-of-date, but it's fascinating to see just how little control we have over who we are.