The Big Five is a big deal because, out of the major personality inventories floating around, it is the only one that was created empirically, as opposed to The Enneagram and the MBTI. Psychological researchers took major adjectives out of the English dictionary, under the assumption that, if people are a certain way, language should eventually reflect that truth. They then had survey-takers sort all those adjectives into different categories, putting in one pile all the words that meant the same thing. It took a lot of math (trade secret: most of psychology is statistics), but eventually psychologists determined that those adjectives fell into 16 categories, which with even more math got refined into a basic five (with some researchers maintaining that each trait has anywhere from two to six subtraits). Whether the 16-factor model or the 5-factor one is more accurate is still being debated by academic psychologists today.
- Openness to Experience determines an appreciation for variety of experience. A Cloudcuckoolander is likely to score high on Openness, whereas lower scorers prefer the traditional and familiar. It relates also to creativity, appreciation of art and imagination; according to what English-speakers say about themselves, both stem from a single central trait. Perhaps unsurprisingly, people who score highly on this trait are also likely to be politically liberal (or occasionally libertarian); some research suggests a positive correlation with high levels of estrogen and dopamine and a negative correlation with serotonin.
- Conscientiousness describes how likely someone is to act with self-discipline, responsibility and an interest in achievement. Essentially it describes self-control, from the "Allergic to Routine" types on the low side to the "workaholic Clock King" types on the high side; some research suggests a positive correlation with high levels of serotonin and a negative correlation with dopamine.
- Extraversion describes a sense of exuberance and willingness to be involved with the world around them: are you energized by being around other people, or does it tend to tire you out? There is actually a physiological explanation for this phenomenon: there are differences between the brain of an Introvert, which is more likely to be overwhelmed by excess stimulation, and that of an Extrovert, which is more likely to be bored by lack of it. Having said that, the Big Five's version of Extraversion also correlates with energy and positivity.
- Agreeableness measures whether you're a Friend to All Living Things or Properly Paranoid. People who score high on Agreeableness are pleasant, cooperative and willing to compromise (but might have trouble putting their foot down when necessary). People who score low on Agreeableness are suspicious of others' motives and place their own interests first. As with Openness, high scorers tend to be more liberal, but libertarians are generally more similar to conservatives; as it seems to be positively correlated with high levels of estrogen and negatively correlated with testosterone, women typically score higher than men.
- Finally, Neuroticism, sometimes called Emotional Instability, measures how well you deal with adversity. People who score high on Neuroticism are easy to knock out of equilibrium; prone to anxiety, depression, stress and so forth; and more likely to overreact to bad news. Low Neuroticism scores indicate stability, calm, and ease at shrugging off negative feelings. This does not mean they feel lots of positive feelings (that's what Extraversion is for); an Introvert with low Neuroticism just doesn't feel much at all. Women tend to score higher than men.
The Big Five test does not provide "types" that comprehensively describe all the different combinations, although John Johnson of Pennsylvania State University has devised a system based on the two most pronounced axes. Partially this is because there would be 32 of them and that would be too much work. Partially this is to avoid pigeonholing; what you score is what you score, and psychologists try not to stereotype. Partially it's because, as with the Myers–Briggs, you can score high on one scale but low on another, and a person with high Extraversion and high Openness is not the same as someone with high Extraversion but only some Openness. But mostly it's because the scales don't combine. If you are Extroverted and Disagreeable, that doesn't make you a Grumpy Old Man, because every adjective you can use to describe a human being is already associated with one, and only one, scale. If you are a Grumpy Old Man, it's because being Extroverted or Disagreeable already means being one.
Obviously, this test is not perfect. For one, it was a textual analysis of the English language, and thus might not be valid for speakers of other tongues. (Translations into other languages have met with varying levels of success; one culture flat-out doesn't correspond to it at all.) For two, there are a number of traits that the test simply doesn't account for, like manipulation, sense of humor and religiosity. As mentioned, some people still prefer the old 16-Factor theory, while others advocate for the 6-factor HEXACO model. And finally, pure general intelligence (the so-called "IQ") seems to correlate with Openness To Experience, which psychologists have flat-out admitted must be a mistakenote . But, again, it's the only personality inventory that was devised by actually inventorying personalities.