The idea behind quantifying personality is deceptively simple: personality refers to predictable differences in behavior between people. Those differences should be reasonably reliable. That is, they ought to hold constant across different types of situations. Those differences should also be reasonably stable, which means they should be consistent over time.
For example, you might score high on the openness factor if you answer “yes” to questions like “I spend time reflecting on things,” and you might score low on the extraversion scale if you answer “no” to questions like “I talk to a lot of different people at parties.” According to personality theory, your answers to those questions shouldn’t change all that much as you grow older, nor should they be different if you complete the survey at home or at the office or at a shopping mall.
Based on this definition of personality, it should be obvious that personality is not limited to humans. Indeed, animal behavior researchers are also interested in defining and quantifying personality. If measuring and describing personality is complicated for humans, it becomes vastly more so for animals. How are these individual differences in predictable responses measured, and classified? How are they even identified? What measurements should be used, and what traits do they measure? In a new paper in press in the journal Behavioural Processes, Noelle M. Watanabe and colleagues from the UCLA Departments of Ecology & Evolutionary Biology and Psychology explore these questions using an unlikely animal model: the Caribbean hermit crab (Coenobita clypeatus).