Confirmation Bias (Cognitive Bias #4)

This is our uncanny tendency to search for, or interpret, information that agrees with our preconceptions. For example, if you suspect that a certain person is a certain way (whether it be evil, lazy, or perfect), you will tend to notice and interpret that person's behaviors in such a way that support your belief, and consider evidence to the contrary as the result of errors of your own perception and judgement, or anomalies.

This is why, in the scientific method, it is useful to conduct an experiment in such a way as to attempt to disprove your theory, rather than prove it.  In order for something to be true, it has to also be falsifiable.  In other words, there has to be a way to prove it to be false, and those ways have to be tested, in order for it to be considered true. 

The best way to help correct for this bias is to be open to contradictory evidence and to test our hypothesii by attempting to prove them wrong.

Read more: Confirmation Bias [Skeptics Dictionary]