But where do personalities come from, and why are they so different?
Over the last 25 years, psychologists have found that personalities coalesce around five basic traits, dubbed the Big Five. Everyone can be described as having varying levels of agreeableness, conscientiousness, neuroticism, extroversion and openness to experience.
Contrary to common perception, people aren't confined to certain personality types. Usually, no one is entirely an extrovert or an introvert, or a total neat freak or a slob. While a minority may be at the extreme ends of a trait, most people are somewhere in the middle.
"We know conclusively from the research that people just aren't organized into types," said Christopher Soto, a psychologist at Colby College in Maine. "Every personality trait is a continuous dimension. You can be very high or very low, and most people fall somewhere in between."
(To see where you are among the Big Five, or other aspects of personality, take these personality quizzes that were developed by research psychologists.)
Personality isn't confined to humans, either. Research finds that animals from ants to apes all have personalities, described also by the Big Five. The universality of personality points to an evolutionary origin.
"Animals and humans all have common problems of survival," said Frank Sulloway, a psychologist at the University of California, Berkeley. "Those common problems of survival are spelled out beautifully in the Big Five, and that's why there's so much continuity in human personality and animal personality."
For example, conscientiousness involves behaviors like planning and deliberation, which are important among primates and other mammals for taking care of offspring, selecting mates and living in groups. Being neat and orderly — aspects of conscientiousness — also has evolutionary advantages. Orb-weaving spiders that spin tidy webs catch more prey, according to research published in 2015 in the journal Integrative & Comparative Biology. Darwin's finches that build neater and well-camouflaged nests attract more mates, scientists reported in 2009 in the Biological Journal of the Linnean Society. And scientists reported in 2011 in the journal Animal Behavior that more hygienic bees that remove more dead colony members reduce risk of disease, gain more weight and reproduce more.
Some of these behaviors may seem primitive, but they're nevertheless manifestations of personality. "In the broadest sense," Sulloway said, "personality is the expression of all of the behaviors that we and other animals exhibit that allow us to function adaptively in the world."
Evolution can also explain why personalities vary so much. Depending on the situation, each of the Big Five can be advantageous. For example, Sulloway said, agreeableness is great for relationships. But if a lion were charging at you, you'd be better off with a less agreeable and more aggressive personality leaning. Because the world is so unpredictable, every aspect of each personality trait could be useful at different times, so instead of evolving a single type of personality that's optimal for every situation, we're left with a wide variety.
"There is no single solution to what personality and behaviors you should be expressing," he said.