Sarah’s favoritism toward certain people came in handy in her groundbreaking participation in tests that lead to a subfield of inquiry known in psychology and philosophy as “theory of mind.” Sarah helped David Premack and co-author Guy Woodruff answer the title question of their 1978 paper “Does the chimpanzee have a theory of mind?” in the affirmative.

To have a theory of mind is to be able to attribute purpose, intention, beliefs, desires, and other attitudes to both oneself and another person or animal. In order to test whether Sarah could understand that people had thoughts that differed from her thoughts, she was presented with short video tapes where a human actor in a cage was trying to perform a task, like trying to get some bananas that were inaccessible. After watching the video Sarah was shown two pictures, one that would allow the actor to reach his goal (a box) the other not ( a key). She successfully solved the problems for the actor.

But there was some concern that she was putting herself into the position of the actors, which would be a pretty exciting cognitive feat on its own, but wouldn’t show that she attributed attitudes to the actors. So she was presented with more videos, one in which the actor was her favorite caretaker and another in which the actor was someone she didn’t really like. Sarah selected the right responses more often for the actor she liked, and the wrong responses for the actor she didn’t much care for.

Sarah’s career established that not only do chimpanzees have complex thoughts, but also distinct personalities with strong preferences and prejudices. But this is just part of her remarkable life story. As she grew older she helped a diabetic chimpanzee named Abby, who she was living with, remember to get her medication . She was a loving, yet stern, aunt-like figure to a pair of young chimpanzees, Harper and Emma, and she helped Henry, a male chimpanzee who came from a situation of terrible abuse, get along with other chimpanzees.

Since the time that Sarah was thought to have established that chimpanzees know what others might want or need, a growing number of investigators have tried to figure out if other animals have a theory of mind. Though there have always been skeptics, studies have suggested that crows, jays, ravens, other apes, monkeys, and maybe dogs, may know what others are thinking. In social animals, being able to glean what others might be thinking is a good strategy for getting along. For chimpanzees living in sanctuaries, it can facilitate care.