It is the study of the mind. The examining of behavior. The parts that make people who they are in a given situation. It is also an entire line of study in school, and for many, it is a soft science. In pop culture, the main protagonist in the TV series Bones, Dr. Temperance Brennan (played by Emily Deschanel) often insulted psychology as not being a real science. (I was a big fan of the first 4 seasons of the series.)
As a person with a science background, I agree that psychology can be a bit squishy in the middle. People are rarely rational and will do things external observers would not expect. Heck, people do things most of the time without consciously thinking about the underlying reasons for their behaviors. But, that inability to predict exactly what someone will do doesn’t make psychology wrong. It just makes it like weather forecasting: a tool that does not have all the variables. However, when enough of the most significant variables are known, psychology allows for accurate predications and can help someone see the butterfly within the caterpillar.
Psychology for writers
Writers need a good grasp of psychology. It helps in the identification of motives for characters and making the protagonist and antagonist act in a more consistent and realistic manner. Understanding the mental condition of our abused pawns gives us the means to manipulating them through our narrative without triggering the dreaded breaking of the suspension of disbelief.
As a natural introvert—who has learned to function as an extrovert when needed—I spend a fair amount of time watching other people. Coupled with a lot of self-reflection, mental time alone with my characters, and some basic psychology courses in college, I attempt to get into the minds of my characters and understand them at a deeper level. Sometimes I don’t dig down deep enough, and then I struggle to tell a particular story. When I take a step back to figure out my own mental state, I realize that I have not put my character on the couch long enough to learn their little quirks. After a bit of psychoanalysis, I can get back in front of the keyboard and make their lives miserable instead of mine.
When did I start doing this?
I think I have always been on the curious side of looking at motivations. A big factor comes from my formative years, where Isaac Asimov injected the notion of treating psychology as a hard science for me in his Foundation series with his concept of Psychohistory. Which is the idea that one can predict the future if there is a large enough body of people to use as a model and you have a powerful enough computer to process the math behind it.
This broader concept of psychology at a societal, or even species, level comes to play in my novels when I have to predict how the people of a given country might act when certain things happen. Or, conversely, what trigger might my characters might need to invoke to cause society to act in a certain way. It becomes an enormous game of what-if scenarios.
Of course, that leads into psyops and manipulating large groups of people to achieve militaristic outcomes. I would like to say my time as an author has helped me recognize some of these operations in practice in the real world and that I don’t fall for them more often than not. However, the internal psychologist in me wants to say that might reflect a bit of hubris.