My favorite tool for predicting and understanding behavior is the principle of path of least resistance.
First time I encountered the term, it was in the context of physics; imagine the path a body of water takes when it flows down a hill and you’ll get the general idea. Apparently, it is analogous to the principle of least effort which is another good name for the same concept, although I feel that the term “resistance” is broader and thus applicable in a wider variety of contexts.
When used in the context of behavior, the principle states that people always takes the option that offers them the least resistance at the present; Not the the most desirable option, not the easiest option but the option that feels the most convenient when the decision is up.
This make it easier to understand many instances of human behavior, especially the irrational ones. You may want to lose weight but avoid exercising. You may want to get good grades but neglect your homework. You may want to achieve your goals but fail to put in the effort.
This is not to say that we will always take the easy way out in favor of the more fruitful. In fact any of us can summon recollections of both kind of behavior. We may not be able to change the fact that we’ll take the path of least resistance but we can affect how resistant each path is. After all resistance of a path is predominantly relative; Our perception, imagination and experiences shape these paths.
For example it is hard to quit smoking. When the decision is about smoking a single cigarette, it is easy to give in. But if you can transform that decision into a question of smoking thousands cigarettes and accepting the consequences that accompany that, you might be able to tip the scales.
In the future I might expand on more examples of this or touch on how the principle is useful in other contexts such AI and UX design.