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Studying personality is hard work. Ideally, we'd be there to capture each and every moment, each unique detail about that person as they move through the world and interact with those around them. But we just can't be there all the time.

Yeah, it's hard work. A well-known social psychologist once wrote a long section on social psychology, and ended it by talking a bit about personality, and explained that he wasn't doing the personality yet because it's so much harder. If you think of two people you know, no matter who they are, you don't get them mixed up, psychologically. Everybody seems different than everybody else. And so you have to, in a complete psychology of a person, come up with a study of individual differences, a systematic universal statement, and also capture the idiosyncrasy of the individual.

What psychologists who study personality traits have sought to do is to use certain terms to describe the differences in behavior between individuals. And in doing so, they've given us the trait approach to personality.

Are there fundamental, underlying dimensions? The search for that has been keeping us going for quite awhile— for 2,500 years.

In psychology, the word trait took on a certain meaning, in particular in US psychology through the work of Gordon Allport— originally in the 1930s— who reasoned that there were certain consistencies in a person's style of behavior. Those consistencies may reflect some sort of inner psychological quality. Those consistencies then are the person's trait. And Allport and a student— Odbert by name— at one point in the 1930s cataloged the traits.

Allport took the dictionary and read it, and had his colleagues read it and find all the words in the dictionary that describe stable characteristics of people. There were about 16,000 such words. The idea behind that was if it's an important dimension, people will have words to describe it. And so he said, what are the important words?

So Allport Odbert created the problem that there's thousands of these trait words, and you want to simplify them. About as simple as you can get is a model by the British psychologist, Hans Eysenck, who suggested that a normal, everyday personality— two primary variations in extra version and neuroticism— are a kind of a map. Every individual could be described as being somewhere along dimensions of extraversion and neuroticism.

Those two broad brush dimensions seem to work quite well in describing how people differ. Whether there's a single mechanism for extraversion is probably not the case. It's probably multiple mechanisms.

So most people realize that, useful as they were, the two dimensions of Eysenck missed some things. So the suggestion from many psychologists today is that a model with five of these individual differences does a good job of describing main individual differences.

If you want to understand what you're going to do, I need to understand what you want to do, how able you are to do things, and your stability in reacting to those situations. I need to understand all of those— the Big Five.

In the five factor model, we are presented with super categories of human personality— conscientiousness, agreeableness, neuroticism, openness to experience, and extraversion. Use the acronym CANOE to help you remember them. The big five model is helpful in placing individual characteristics on spectra that we can apply to large populations of humans. When you are more of one thing, it does not necessarily mean you are less of another. Each person moves through the world with a unique brew of these traits. It's through teasing out the nature of that brew that trait theories of personality seeks to understand the individual.

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In extraversion, a tendency to approach people, a tendency to have positive affect, be happy, cheerful. Somewhat interested in dominance and leadership versus people who are not so— not that they're unhappy, but there's not— don't experience positive affect as much, we call introverted.

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It's once again, it's a dimension. And so people range from never doing what they say they'll do to the extreme levels, which we call obsessive compulsive, that always do what they say they'll do. So all of these are dimensions rather than types.

More than any other, the big five model seems to best capture personality. Some studies have shown that these descriptions show up across a wide range of participants, including those of different ages, nations, and languages. And research continues into the replicability of these factors across cultures.

The hope for a model like the five factor model is that it's going to apply widely. The perfect payoff would be that you could go anywhere in the world and find that there were these five and no other important individual differences in the local language.

But where do these traits come from? More modern research has added the elements of genetic inheritance and neuroscience to the mix. Eysenck originally speculated that different levels of cortical arousal may underlie differences between extroverts and introverts. Behavioral and physiological research now mostly supports the speculation, and it's commonly believed that some core differences in personality may be based on variations in sensitivity of brain activity in certain regions.

So for 21st century neuroscience and genetics, if you enter into a model like the five factor model— and sometimes you get pluses and minuses— you get somethings that support the model but some things that, at the theoretical level, don't look much like it at all. So what supports the model is the core presumption that there's something biological behind these traits. The simplest support for that is that if you get some identical twins, including that special case of ones that are separated at birth, and you measure them on these traits, they are similar to each other far more than random people, far more than fraternal twins. There's something biological there.

Much of contemporary neuroscience and genetics is telling us that the biological system changes as a result of experience. And frankly, that at a theory level, was not anticipated by the trait theories, which would have to be seriously reworked from their 20th century versions to take into account the changes at a core biological level that can occur as a result of experience.

So however you study personality, make sure you're taking into account the whole picture, inside and out.

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