Here are a few psychological studies that have profoundly affected the way we understand human behavior and mental processes. Appreciating how much the environment can change our behavior can help you create more realistic tension and troubles for your characters.
Following the Nuremberg trials of Nazi war criminals, psychologists tried to understand what human behaviors made the atrocities of WWII possible.
What Milgram found was that people, regardless of their sex, education, or values, obeyed people they perceived as being in charge in spite of their belief that they were shocking and maybe even killing another person (65% of subjects administered lethal-level shocks).
Though most of us would like to believe we would defy authority, Milgram has proved that authority is far more powerful than we think it will be. And among those subjects who finally defied the researcher, not one left his seat to help his victim until the researcher gave permission.
Phil Zimbardo conducted an experiment in which volunteers were randomly selected to be either "guards" or "prisoners" in a makeshift prison in the basement of a Stanford University building. What was supposed to be a 2-week experiment on authority and obedience became so real that least two "prisoners" had breakdowns, and the guards came up with increasingly sadistic punishments for disobedience. The experimenters, the subjects, their parents, and even the chaplain got so wrapped up in their roles that an outsider had to point out that it needed to stop. The experiment was ended at 6 days.
Solomon Asch demonstrated that people will often choose an answer they know is wrong because other members of the group do so.
Beauty Check 
has compiled some of the most compelling findings on what we find beautiful and how perceptions of beauty affect the ways we interact with other people.
For example
so cute) The site has lots of pictures so you can see what they're talking about. So no matter what your character's like on the inside, the outside is going to affect the way others deal with him.
Too many writers blame their villains' sadistic behaviors on "insanity" or "mental illness" when in fact the reasons are almost always far more complex. Psychologist David Buss has explored, among other things, how and why people have evolved to kill.
Arguably the foremost evolutionary psychologist in the United States, Buss works at the University of Texas. His website includes a long list of studies you can read. Buss has also developed compelling theories on why women love bad boys and men always want a younger woman and why phenomena like sexual harassment occur. Evolutionary psychology attraction theories have been bastardized into something called Ladder Theory, which is a crass repackaging of the material to make it more appealing to the cynical among us. Interestingly, Ladder Theory is just the tip of a very large iceberg of recent electronic and print books for men on landing one-night stands.