Motivated reasoning is the phenomenon in cognitive science and social psychology in which emotional biases lead to justifications or decisions based on their desirability rather than an accurate reflection of the evidence. It is the "tendency to find arguments in favor of conclusions we want to believe to be stronger than arguments for conclusions we do not want to believe". People can therefore draw self-serving conclusions not just because they want to but because the conclusions seemed more plausible given their beliefs and expectancies.
Motivated reasoning is similar to confirmation bias, where evidence that confirms a belief (which might be a logical belief, rather than an emotional one) is either sought after more or given more credibility than evidence that disconfirms a belief. It stands in contrast to critical thinking where beliefs are approached in a skeptical and unbiased fashion.
It can lead to forming and clinging to false beliefs despite substantial evidence to the contrary. The desired outcome acts as a filter that affects evaluation of scientific evidence and of other people.