The theory of motivated reasoning does not, in and of itself, explain why we might be driven to interpret information in a biased way, so as to protect and defend our preexisting convictions. And, inevitably, humans will have a great variety of motivations, ranging from passionate love to financial greed.
What's more, the motivations needn't be purely selfish. Even though motivated reasoning is sometimes also referred to as "identity-protective cognition," we don't engage in this process to defend ourselves alone. Our identities are bound up with our social relationships and affiliations -- with our families, communities, alma maters, teams, churches, political parties. Our groups. In this context, an attack on one's group, or on some view with which the group is associated, can effectively operate like an attack on the self.
That's where politics comes in. Our political, ideological, partisan, and religious convictions -- because they are deeply held enough to comprise core parts of our personal identities, and because they link us to the groups that bulwark those identities and give us meaning -- can be key drivers of motivated reasoning. They can make us virtually impervious to facts, logic, and reason. Anyone in a politically split family who has tried to argue with her mother, or father, about politics or religion -- and eventually decided "that's a subject we just don't talk about" -- knows what this is like, and how painful it can be.
And no wonder. If we have strong emotional convictions about something, then these convictions must be thought of as an actual physical part of our brains, residing not in any individual brain cell (or neuron) but rather in the complex connections between them, and the pattern of neural activation that has occurred so many times before, and will occur again. The more we activate a particular series of connections, the more powerful it becomes. It grows more and more a part of us, like the ability to play guitar or juggle a soccer ball.
So to attack that "belief" through logical or reasoned argument, and thereby expect it to vanish and cease to exist in a brain, is really a rather naïve idea. Certainly, it is not the wisest or most effective way of trying to "change brains," as Berkeley cognitive linguist George Lakoff puts it.
We've inherited an Enlightenment tradition of thinking of beliefs as if they're somehow disembodied, suspended above us in the ether, and all you have to do is float up the right bit of correct information and wrong beliefs will dispel, like bursting a soap bubble.
Beliefs are physical. To attack them is like attacking a part of a person's anatomy.
Nothing could be further from the truth. Beliefs are physical. To attack them is like attacking one part of a person's anatomy, almost like pricking his or her skin (or worse). And motivated reasoning might perhaps best be thought of as a defensive mechanism that is triggered by a direct attack upon a belief system, physically embodied in a brain.