I recently read a truly interesting book about behavioral economics called Irrationally Predictable. It's a fascinating read about why people make the decisions they make - even when they don't appear to be in our own self interest.
One chapter in particular focuses on the different ways social norms influence human behavior. In a fascinating study, the participants demonstrated that they were more highly motivated when doing work for free (as a favor or charitable act) than they were if they felt they were being underpaid for their time. If the option was a fair market wage, a low wage, or free, the people who performed the task for free were actually the most productive of the three groups. It showed how strongly we are influenced by social norms that encourage us to be neighborly and good.
Coming at it from a writing viewpoint, it's an intriguing way to approach character and conflict. Social norms are all about balance. As soon as one party takes advantage of the social relationship, it begins to break down and market norms take over.
For example, let's say my heroine loves doing things for her family. She gets a strong sense of validation from it, even though she is busy and overworked and doesn't really have the hours to waste - but to her they aren't wasted hours. Until something happens and she realizes that her family is taking advantage of her kindness. Suddenly her time is no longer weighed in "but I want to do this for you" and it starts to be measured in all the other ways she could spend her time. It becomes a tug-o-war between the market norm (how much she gets paid for her time at work) and the social norm (the satisfaction she gets out of helping her family). And any time you have an internal tug-o-war, you have conflict.
There were at least a dozen additional experiments in the book that evaluated other aspects of human nature. One focused on the way humans become irrational when aroused or caught up in heightened emotional states. That one is particularly tricky. Certain parts of our brains definitely power down when we're turned on or fired up, but how do you write that without tripping into the 'Too Stupid to Live' heroine or the Alpha Asshat hero? It's a fine line between the Irrational Reality and the Logical Credibility that will keep your readers from chucking the book across the room.
I could probably go on about Irrationally Predictable for a few more days - especially if I accessed the Number Nerd part of my psyche and started looking at the way it applies to market trends - but I'm going to shut up now with one final question for you:
Consider the line between believable irrationality and irredeeemable stupidity in characters. We all do stupid things for stupid reasons, but are our characters allowed to be as irrationally human as we are?