So, let's start with personality. Personality is defined as a person's style of dealing with the world and particularly, their style with dealing with other people. Intuitively, we have a sense of what it is to be a certain personality. You acknowledge that you are a certain type of person, maybe you're shy, or funny, or extroverted, or conscientious, moody, or some way of characterizing it. You could characterize the people you know this way. You can characterize fictional people like Homer Simpson or a famous people you don't know like Kanye West. Based on your experience of their actions, you can make inferences about their personality. But the question is how can we scientifically zoom in and characterize personality? Through the history of psychology, there have been attempts to have different tests, different measures of personality. One way to assess the test is on two criteria, reliability and validity. Reliability involves measurement error. So, reliability is if you test the same person over time, you should get the same result. That's what a reliable test is. If somebody goes in your bathroom scale under 160 pounds and they step off and they step back on and it's 160 pounds again, it's a reliable scale at least based on that test. If they step off and then step on and is 20 pounds lighter, 20 pounds heavier, the scale isn't reliable. A second way to make sense of tests is validity. How well it measures what it's supposed to measure? So, for instance, you can determine if you're in test neurology your personality by the date of your birth. This is a highly reliable measure. You're tested over and over again and you'll always get the same results. But it's not a valid measure because it doesn't work. It doesn't actually measure your personality. If you have access to the Internet, which I assume you do, it's easy to take a lot of personality tests. Recently, I took the personality tests online, "Which superhero are you", and it said that I am Batman. I am dark, love gadgets, and have vowed to help the innocent not suffer the pain you have endured. Now, this isn't bad, but the test is neither reliable nor valid. It's not reliable because when I first took it, I came out as the Hulk, then I took it again, I came in as a Wonder Woman. I kept answering the questions in different ways so I got the answer I wanted. Basically, I wanted to be Batman. It's not valid either because there's no reasonably that it actually taps any deep sense of our nature. We can try to do better. One classic personality test over time is the Rorschach Inkblot. This is a test that's been used for psychiatric cases, for criminal cases, part of clinical psychology training. The idea is that if you properly interpret people's responses to inkblot, what do they see? You should look at this and figure out what you see. Well, it turns out most psychologists believe it has no validity at all. It does not tell you anything about the deep aspects of your psychology. So, it could be used as a game but nothing more. So, what's a more scientific approach? A while ago, the psychologist, Gordon Allport, took 18,000 traits from a dictionary that he viewed as bearing on personality. Now, that's a lot, but it turns out they aren't independent. Many, many words are just different ways of saying the same thing. So, whether somebody is a friendly or sociable or welcoming or a warm-hearted, basically, you're capturing the same trait. So, psychologists then try to zoom it down. The psychologists icing broke it down into two main traits. At first, whether you're introverted or extroverted, and whether you're neurotic or stable. Then, there's a third trait whether psychotisism versus non psychotisism. Basically, are you aggressive or you're compassionate? Cattell, as you see here, broke it down to 16 different factors. Factors like sociable versus non-sociable, bold versus timid, tense versus relaxed, and so on. Now, a lot of modern commentators have come to the consensus that 16 is too many, and two to three are too few. They've zoomed in on the big five. These are neuroticism, stability. Basically, are you a worrying, anxious, self-pitying, vulnerable person or more calm? Extroversion versus introversion, which you know what it means. Openness to experience versus non-open to new experiences. Agreeable versus antagonistic, and conscientious versus undirectednes. Basically, the idea is that the questions you would ask of a new person, somebody you're interesting in to know, will all fall under at least one of these five categories. Are they nice or are they nasty? Are they reliable or are they a flake? Are they nuts or are they stable? Are they bold or are they cautious? Are they a leader or follower? Notice that the first two items on the scale are from icing. Fortunately, for students interested in taking tests, if you switch around the order of these, you get the acronym OCEAN. OCEAN is therefore a nice thing to remember as the big five of personality. It turns out that these really work. So, they're pretty stable over many years. If you test somebody at age 40 and test them again age 50, you will find that they're the same. People agree on them. So, the way you characterize yourself on a scale or roughly fit the way other people characterize you. Your score on a personality test will predict real-world behavior. People who are conscientious on the test tend to have more marital fidelity. People with openness to new experiences changed their jobs more. Extroverts look people in the eye more and have more sexual partners. So, to a first approximation, we've answered part of the question of how people differ by focusing on personality, and say there are five separate dials, each they could be turned to a different setting. Your pattern on those dials is your personality.