You and I both no doubt take some certain amount of pride in our abilities to think rationally. We can take a view of the world and apply objectivity to it, consider facts over emotions, and distrust promises made without proof. We are aware of biases in the world, including our own. If my rough demographic sketch is correct, you probably don’t believe in god either, or at least a knowable god.
This is a good way to exist in the world, but, according to a study in last week’s Science, it's missing something. There’s a force of perception known as “contrast effects” that applies to many animals, as well as humans. This is when our decisions/perceptions become influenced by how things have been in the past, good or bad or otherwise. There is some level of “normal” behavior, but in terms of “perception, cognition, and related performance,” that is heightened or decreased depending on, well, how things have been. Lift something heavy and then lift something light immediately afterward; the light thing will feel lighter than it actually is. That’s the very basic idea.
For the rest of the story: http://motherboard.vice.com/blog/rational-thinking-is-a-lot-less-objective-than-you-thought