Animals are used to study human behavior and psychology because both species exhibit behavior with similar characteristics.  Humans as a group are hard to observe;therefore, researchers have studied animals and their human-like behaviors to deduce what humans might do, think, or feel in the same circumstance or with the same sort of group.  For example, monkeys are the classic example of animals who live in a group, take care of each other in certain ways, and the females care mostly for the young. Many similarities exist  in their group structure which directly relate to human beings; monkeys can be observed to think through a problem with another monkey and follow through with a plan.  Elephants are another group which live according to a fairly tight group structure.  They grieve when another elephant dies just as humans would do and for the same reasons.  Psychologically, animals can be used to deduce human psychology which is why researchers use what is available and what works.