I have a friend who's getting his PhD in Psych. He's a blast to talk to, if you can deal with being intermittently analyzed.
One thing he introduced me to is the idea of a "happiness baseline". The idea is that everyone has a happiness baseline. There's an interesting Times article here about studies that found identical twins tended to rate their happiness similarly (in contrast to non-identical twins), and that there were three personality traits (extroversion, calmness and conscientiousness) that seemed to account for their happiness levels. Another study suggests that whatever calamities befall a person, within a few years they return to their baseline happiness (I wonder if this holds true for winning the lottery and whatnot as well... I would guess that it is).
I think this is an interesting thing to consider with a character, one of the many little psychological tidbits that can help you develop your concept of them. Does your character have a naturally high happiness baseline -- someone who takes everything in stride and stays all "akuna matada"? Or is your character darkly cynical, entertaining to the reader but never quite happy with their life? Or... like most of us... is your character set somewhere in the middle?
And of course, happiness is a great motivator for behaviors; it can motivate your characters to make great changes in their lives, try to keep everything the same, or even create protagonists that threaten their happiness.
Haidt, in The Happiness Hypothesis: Finding Modern Truth in Ancient Wisdom, suggests that happiness is formulaic: happiness = genetic set point + life conditions (things you can't control or that change slowly, like race, gender, wealth etc) + voluntary activities (things you do that are gratifying). What's your MC's set point, their conditions, their activities?
My MC, Lauren, is an overall happy person. Before she's inhabited by the Greek goddess of vengeance, she's also been lucky to have a great set of conditions: loving parents, a great education, lots of opportunities, true love, and a section of the world pretty free from violence, racism and sexism. But then, of course, I have to ruin her whole life to make for an entertaining book. But! I am pretty sure Lauren will not only survive my novel with her sense of humor intact and make herself a happy ending, but that she'll find a way to make it back to her old happiness level -- even if her life will never look the same.
And of course... but it has to get personal...
What's your happiness baseline? I think I have a naturally high baseline, since I have a pretty sunny outlook (in fact, when I went to Officer Candidate School for the Marines, I was constantly screamed at for smiling... and for telling one of the instructors that I saw the positive in everything!).
COM O DEDINHO NO CUZINHO
7 hours ago