In psychology, the false consensus effect, also known as consensus bias, is a pervasive cognitive bias that causes people to “see their own behavioral choices and judgments as relatively common and appropriate to existing circumstances”[1]
When we meet somebody new, we assume they think and behave as we do. Some people do, of course, but not everybody. Our attitudes, thoughts, and actions are usually shaped by our family, friends, acquaintances, and life experience, and since everybody has different life experiences and people in their lives, behaviors, thoughts, and attitudes are different from ours. This bias goes a bit further, as you can imagine. Our brains are very powerful supercomputers. When we imagine somebody is dishonest or lacks integrity, our brain tries to affirm our original thought that the person is dishonest and lacks integrity. We continue to look for signals to confirm the bias that was initially thought to be true, whether the bias is true or not. Unfortunately, we hold onto those biases quite firmly, rather than questioning the basis of our belief. Remember, input leads to belief, which leads to attitudes, which leads to actions and then results. If we begin a situation with improper information, we have to be open-minded to listening to other points of view, and listening to possible inconsistencies to our original biases. Think of someone or something that you assumed was true. Now think of somebody you know who disagrees with your "truth." Be open minded, and listen. You may find that you are hanging onto a baseless bias. Or maybe not. At least challenge why you believe what you believe. Always.

[1] Ross, Lee; Greene, David; House, Pamela (May 1977). "The 'false consensus effect': An egocentric bias in social perception and attribution processes". Journal of Experimental Social Psychology. 13 (3): 279–301. doi:10.1016/0022-1031(77)90049-X.