Psychology
Some types of experiments cannot be done on people because it would take too long, cost too much money, risk people's health, or be unethical. There are also other ways psychologists study the mind and behavior scientifically, and test their theories. Psychologists might wait for some events to happen on their own; they might look at patterns among existing groups of people in natural environments; or they might do experiments on animals (which can be simpler and more ethical to study).