PERSONALITYPersonality categories
Somewhat trickier than establishing the consistency and predictive
power of personality has been the age-old conundrum of how exactly to
categorize the different personality traits. The idea that each of us has
one of four temperaments or humours – phlegmatic, choleric, sanguine
or melancholic, depending on the predominance in the body of either
phlegm, yellow bile, blood or black bile – dates back at least as far as the
Ancient Greeks and formed the basis of Hippocratic medicine. Over
the last hundred years, psychologists started to develop theories with a
more scientific basis. The model devised by Hans Eysenck (1916–97), for
example, had an extrovert/introvert dimension, a neurotic/stable dimen-
sion, and later on, a psychotic/socialization dimension. Raymond Cattell
(1905–88) proposed sixteen personality factors, including warmth, vigi-
lance and dominance. By the end of the last century, a gratifying consensus
emerged, based on the idea of stripping out any redundancy in different
personality traits – a process known more formally as factor analysis.
For example, given that being sociable and being enthusiastic nearly
always go together, it makes sense to collapse these into a single trait.
Applying this idea to a multitude of different personality characteristics,
psychologists today largely agree that there are five main personality
traits – known as the Big Five – that can’t be reduced down or lumped
together any further. These are Extroversion, Neuroticism, Conscien-
tiousness, Agreeableness, and Openness (to identify your own scores on
these factors see p.184). All are continuous dimensions, rather like height
or weight. The fact that so much variation in these traits has persisted
in the human genome suggests that, in the past, a mix of environmental
circumstances allowed different personality types to thrive. Underlying
variation in each of the Big Five traits is the functioning of a particular
brain system or systems.
Extroversion describes how motivated a person is in the pursuit of
reward. High scorers on this dimension tend to be doers and seekers.
They go to more parties, have more sexual partners, and are more thrill-
seeking than low scorers. Put an extrovert in a brain scanner, show them
positive images, such as of an erotic couple or a delicious meal, and their
brain will fire up far more explosively than a low scorer on this dimen-
sion. The implication is that the reward pathways of their brains are
more sensitive. It’s worth noting that a low scorer on the extroversion
trait is not necessarily sad and withdrawn. Rather it’s that they’re less
affected by and driven towards positive reward.