My sister Katie was born when I was three, when my own puny brain was still relatively fresh. We had the same parents, grew up at the same time, in the same place. It was the 1980s in a small isolated Welsh valley community. Overall, we had very similar environments, and very similar DNA.
You might expect us to have very similar personalities. This is the opposite of what happened. My sister was, to put it mildly, a hyperactive nightmare, whereas I was typically so placid you had to poke me to make sure I was conscious. We’re both adults now, and still largely different. I’m a neuroscientist; she’s an expert cupcake maker. This may seem like I’m being condescending, but I’m really not. Ask anyone what they’d prefer: a discussion on the scientific workings of the brain or a cupcake. See which one is more popular.
The point of this anecdote is to show that two people with very similar origins, environments and genetics can still have vastly different personalities. So what chance does anyone have of predicting and measuring the personalities of two total strangers from the general population?
Take fingerprints. Fingerprints are basically the pattern of ridges in the skin at the end of our digits. Yet, despite this simplicity, almost every human on earth has unique fingerprints. If surface patterns of small patches of skin offer enough variety for everyone to have his or her own exclusive set, how much more variety is possible with something that is the result of countless subtle connections and complex features of the human brain, the most complicated thing in the universe? Even to attempt to determine someone’s personality with a simple tool like a written test should be utterly futile, a task akin to sculpting Mount Rushmore with a plastic fork.
However, current theories argue there are predictable and recognizable components of personalities, labeled “traits,” that can be identified via analysis. Just as billions of fingerprints conform to just three types of pattern (loops, whorls and arches) and the vast diversity of human DNA is produced by sequences of just four nucleotides (G, A, T, C), many scientists argue that personalities can be viewed as specific combinations and expressions of certain traits, shared by all people. As J. P. Gillard said in 1959,2 “An individual’s personality, then, is his unique pattern of traits.” Note how it says “his”; this was the 1950s, and of course, women were allowed to have personalities only from the mid-1970s.
But what are these traits? How do they combine to form a personality? Arguably the most dominant approach at present is the “Big 5” personality traits, which argues that there are five traits in particular that make up a personality, in the same way that multiple colors can be made by combining red, blue and yellow. These traits are often consistent across situations and result in predictable attitudes and behaviors in an individual.
Everyone supposedly falls between two extremes of the Big 5 traits:
Openness reflects how open to new experiences you are. If invited to see a new exhibition of sculptures made out of rotten pork, people at the extremes of openness may say, “Yes, definitely! I’ve never witnessed art made of rancid meat, so this will be brilliant!” Or, “No, it’s in a different part of town from where I usually am so I won’t enjoy it.”
Conscientiousness reflects the extent to which someone is prone to planning, organizing, self-discipline. A very conscientious type might agree to attend the rotten-pork exhibition, after working out which would be the best bus route with alternatives in case of traffic disruptions, and also getting a tetanus booster. A non-conscientious type would just agree to meet there in ten minutes, not ask permission to leave work early and opt to follow their nose to find the location.
Extroverts are outgoing, engaging, attention-seeking, while introverts are quiet, private and more solitary. If invited to the rotten-pork exhibition, an extreme extrovert will attend and bring their own hastily made sculpture to show off, and end up posing alongside all the exhibits for their Instagram account. An extreme introvert wouldn’t talk to someone long enough to be invited.
Agreeableness reflects the extent to which your behavior and thinking is affected by a desire for social harmony. A very agreeable person would surely agree to attend the rotten-pork sculpture exhibition, but only as long as the person inviting didn’t mind (they don’t want to be a bother). Someone totally lacking in agreeableness probably wouldn’t be invited anywhere by anyone in the first place.
A neurotic person is invited to a rotten-pork sculpture exhibit and they decline and explain why in exquisite detail. See: Woody Allen.
Unlikely art exhibitions aside, these are the traits that make up the Big 5. There’s a lot of evidence to suggest they’re quite consistent: a person who scores high on agreeableness will show the same tendencies in a wide variety of situations. There is also some data linking certain personality traits with specific brain activity and regions. Hans J. Eysenck, one of the big names in personality studies, claimed that introverts have higher levels of cortical arousal (stimulation and activity in the cortex) than extroverts.3 One interpretation of this is that introverts don’t require much stimulation. Extroverts, by contrast, want to be excited more often, and develop personalities around this.
Recent scanning studies, like those by Yasuyuki Taki and others,4 suggest that individuals demonstrating neuroticism show smaller-than-average areas such as the dorsomedial prefrontal cortex and the left medial temporal lobe including the posterior hippocampus, with a bigger mid-cingulate gyrus. These regions are implicated in decision-making, learning and memory, suggesting a neurotic person is less able to control or suppress paranoid predictions and learn that these predictions are unreliable. Extroversion showed increased activity in the orbitofrontal cortex, which is linked to decision-making, so perhaps because of this raised activity in the decision-making regions, extroverts are compelled to be active and make decisions more often, leading to more outgoing behavior as a result?
There is also evidence to suggest there are genetic factors underlying personality. A 1996 study by Jang, Livesley and Vernon using nearly 300 pairs of twins (identical and non-identical) suggested that the heritability of the Big 5 personality traits ranged from between 40 percent to 60 percent.5
What the preceding paragraphs boil down to is that there are some personality traits, specifically five, that have a large body of evidence behind them and appear to be associated with brain regions and genes. So what’s the issue?