These experiences are a function of the environments we encounter. As previously mentioned, for other physical phenotypic traits, environment captures a lot, including nutrition and socio-economic class, but also things that made us nervous, scared, happy or excited in the past. And because we learn, many of the environments that impact the wiring of neurons within our brains are not random. As we develop, we learn what makes us ecstatic, and what makes us fearful or uncomfortable, and we tend to select environments we feel comfortable in. If you are scared of heights, you won’t become a mountaineer. I don’t enjoy dance clubs and so actively avoid them. My experience of dance clubs is consequently limited, and they have contributed little to who I am.
Personality traits, like any other phenotypic trait, are determined by genes, chance and the environment, but they are harder to study because we do not understand the mechanistic cause of traits like introversion or extroversion. Unlike my poor eyesight, where scientists understand the genetic error and how it manifests, we do not have a good grasp of which brain structures equate with outgoingness or humour. Without a mechanistic understanding of how brain structure maps to personality we must rely on statistical correlations, but this too is not straightforward, as my next example demonstrates.
Sonya has a fear of sharks and is never comfortable going into the ocean. She tells me that her fear started after seeing two of her friends swimming in the shallows and spotting a large shark swimming towards them. She shouted and waved a warning and they safely swam ashore, with the shark turning back to the depths. There is no doubt that the experience was frightening, and for a few seconds she was not sure how it would play out. Nonetheless, if that event hadn’t happened would she be unfazed by the idea of swimming in the ocean? Was she already a little nervous about entering the sea before this event? We can never know with certainty because a version of Sonya who did not witness the shark close encounter does not exist, and so we cannot make such a comparison. Because each of us is unique, we cannot do the experiment where we compare versions of ourselves that did and did not experience a particular event.
Even if my wife had an identical twin sister who had not experienced the shark close encounter and who was happy to swim in the ocean, I still could not say for certain that the day on the beach when her friends could have been injured or killed was the cause of Sonya’s fear. Some other differences in the experiences of identical twins, or even apparent randomness during development in the womb or early childhood could be the cause of their different views of the safety of the sea.
Although scientists cannot attribute cause to effect within an individual in terms of why they have the personality they do, they can draw insights by comparing populations of people. If I were to identify 1,000 individuals who had observed their friends experience a close encounter with a shark, and 1,000 who had not, I could measure their levels of anxiety when I take them to the ocean and ask them to swim. I might do this by measuring changes in levels of adrenaline or cortisol, hormones associated with stress, in their blood. If the two groups of individuals did not differ in other ways, I might be able to conclude that seeing a shark swim towards your friends was sufficient to induce a fear of entering the ocean. I would reach this conclusion if individuals in the close-encounter group were, on average, more reluctant to enter the water than those in the non-close-encounter group. To draw this conclusion, the two groups must be as well matched as possible for all aspects of experience, biology and background, other than having seen their friends encounter a shark while swimming. If everyone in the close-encounter group was female, for example, while the non-close-encounter group was all male, I could not distinguish between sex and shark-close-encounter experience in determining fear of the ocean. Perhaps men are just more likely to be scared of the ocean than women and this could explain the difference between the two groups.
Even if my two groups were indistinguishable from one another in all other aspects of their lives except for the shark-close-encounter experience, I could not say with complete confidence that any single individual fears the ocean because she experienced a shark encounter unless all those who experienced a near shark attack were scared of the ocean, and all those who did not were happy to dive into the sea. In most comparisons like this, you do not find that all of one group exhibit one view, while all the second state another. It would be highly likely that some of the thousand individuals who had not experienced a shark close encounter were still scared of the ocean, and vice versa. The best I can say is that, out of a balanced and random sample of 2,000 people, those who experienced a shark close encounter were more likely to fear the ocean than those who had not. I can’t say they will, or will not, fear the ocean, just that they are more likely to do so. Sonya did experience a shark close encounter so it is possible that this has caused, or contributed to, her fear of the ocean, but neither she nor I can ever really know despite her vociferous insistence that this was the cause.
Despite scientists’ inability to link particular experiences to personality traits, all of us have experiences that we believe have moulded our personalities. Some people recount events to explain why they are shy, anxious, funny, talkative, or scared of almost any object you can imagine. For example, a woman from the West of England cannot walk down the frozen food aisle of a supermarket because of a fear of frozen peas. She explains, ‘They tend to just look at me – ganging up on me.’ She wasn’t involved in a terrifying pea prank as a youngster, or a collision with a lorry carrying a shipment of peas, but maybe something triggered it when she was younger. Regardless, the fear is very real and it is part of her personality. With something as complicated as a personality, how can psychologists make progress in working out why we have the characters we do?
The first step is to simplify the problem, and psychologists frequently do this by classifying people on five dimensions of personality: extroversion, neuroticism (or emotional stability), agreeableness, conscientiousness and openness to experience. These are called the big five personality traits, and are part of a framework developed in the 1990s to assign individuals into categories by having them complete a questionnaire. Psychologists like these traits because they capture some of the differences between individuals, and scores for any one individual tend to be very similar if the same individual does the test on multiple occasions spanning months or even years. If acquaintances are asked to complete the survey for someone, there also tends to be agreement between the classifications provided by the subject and her colleagues.
By analysing brain scans, aspects of the environments in which people have lived and genetic sequences of study participants, psychologists have been able to show that, as with many other phenotypic traits, genes, the environment and developmental noise contribute to determining the big five personality traits. Environmental drivers such as nutrition as a child, socio-economic background or the country you are born in tend to explain little of the differences between us. You might think this means that the environment is not important in determining our personality, but that would be wrong. It would be a strange world if all people born rich were shy and all those born poor were not, or if the French were much more anxious than the Germans. The influence of the environment is important, but proving it is hard because individual experiences mould our personalities. Extremely traumatic events, such as abuse as a child, have been shown to impact openness to experience. Fortunately, such traumatic abuse is unusual and most of us do not experience such extreme environments, but that is not to say less traumatic events do not shape who we are. Sonya is convinced her fear of the ocean stems from one experience that lasted less than a minute, and certainly before it she had a job that required her to spend a lot of time in the ocean studying turtles. It is possible she had subconscious anxiety about the ocean that only manifested itself consciously following her close-encounter experience. But scientists cannot prove that.