In most scientific studies the environment is characterized quite coarsely, using variables like class, the religion of the household you grew up in, the number of siblings you have, whether you are the oldest or youngest sibling or somewhere in the middle, nutrition during development, or the cohort of individuals you spent your formative years with. A typical study will correlate values of a phenotypic trait with aspects of the environment such as these. As an example, many studies have found birth order correlates with aspects of personality. In this case, some measure of personality such as anxiety level is the phenotypic trait, and birth order captures an aspect of your environment as a child as it correlates with the amount of attention your parents were able to give you. The firstborn child tends to be conscientious and cautious, while the youngest is more fun-loving, attention-seeking and manipulative. Those in the middle are more rebellious and have a large circle of friends. These differences are thought to arise because the oldest, being the first, gets lots of parental attention. Each middle child receives a little less, with the youngest having to compete the most for attention against its older siblings that still require parental care. Although these effects are undoubtedly real, you shouldn’t read too much into them. Some firstborn children are rebellious and manipulative, and some last-born offspring are conscientious and cautious. Your birth order means you are slightly more likely to have a particular personality trait rather than that you will. Birth order explains only a small part of why people differ, with scientists saying it explains only a small proportion of variation in personality between people.
Although a single characteristic of the environment such as amount of parental attention explains only a fraction of variation in phenotypic trait values such as anxiety level, this does not mean the environment is unimportant. Instead, what such results reveal is that no one aspect of the environment, such as birth order, explains many of the differences between us. When scientists include many measures of the environment into analyses, such as birth order, the type of school you went to, how many overseas holidays you went on with your parents, or the ages of your parents, the total impact of the environment on phenotypic trait values increases. The environment is devilishly complicated, it is hard to quantify, but for many phenotypic traits it plays a key role in how they develop.
The genetic code is long and complex, development is complicated and involves many steps, and the environment is a multifaceted web of lots of different things. As if these drivers of phenotypic differences are not complicated enough, things become even worse when the influence of some aspect of the environment on phenotypic trait values depends upon your genes. Biologists refer to these effects as gene-by-environment interactions.
Smoking is something that is bad for you, and exposing your lungs to cigarette smoke is part of the environment you experience. On average, smokers do not live as long as non-smokers, in part because they are more likely to get various types of cancer, including bladder cancer, as well as the more obvious lung cancer. A smoker is at least three times more likely to develop bladder cancer than a non-smoker. However, not all smokers face the same risk of developing bladder cancer, with the chance of a smoker getting it being dependent upon the alleles they have at a gene called NAT2. Smokers with some genetic mutations at NAT2 are about three times more likely to develop bladder cancer than those with a normal copy of the gene. Your genetic code at gene NAT2 influences the likelihood of poisons in cigarette smoke causing a mutation in a cell in your bladder that makes it cancerous. Your genetic code is said to interact with an aspect of the environment, influencing the chances of developing a nasty disease.
Much of my focus in this chapter so far has been on physical traits such as eyesight or running speed at a particular age, but we also have personality traits. When we describe ourselves, we often mention things such as whether we tend to get stressed, are laid back, or have a fear of spiders. These aspects of our personality are also phenotypic traits, and they too can be influenced by genes, the environment and chance. We are often intrigued by why one sibling is shy and another outgoing, or why we are anxious but our partners are not. The reason is down to nature, nurture and chance, with nurture a function of the environments you have experienced. Despite the complexity of personality traits, psychologists have made progress in exploring the role of genes, chance and the environment on our personalities.
Personality traits come from the structure of our brains and the way brain cells link to one another via synapses. I discussed synapses and how they are involved in the ways our brains work when I explored consciousness. Brains are extremely complex, consisting of between 80 and 100 billion neurons linked by upwards of 600 trillion synapses. It is these linkages that determine how we experience the world, what we remember, how we think and why we have the personalities we do. Psychologists who study the brain do not have a mechanistic understanding of why the networks created by synapses between brain cells make some of us extrovert and others introvert, make some people scared of spiders and others indifferent towards them, or indeed any aspect of our characters. Despite this, psychologists have been able to work out which parts of the brain are associated with particular personality traits, and that genes, chance and the environment all impact our personalities much in the same way that they determine our physical phenotypic traits.
Some of the first breakthroughs in understanding brain structure were achieved by studying how unfortunate accidents have damaged parts of the brain in some patients and how these injuries have altered their personalities. Scientific papers on these cases typically refer to patients by numbers to protect their true identities. In one paper, scientists report how a thirty-year-old man referred to as Patient 2410 suffered a brain bleed towards the front of his brain during surgery. Before the injury his wife described him as short-tempered, often angry, and mopey. The surgery changed him. Afterwards he was much more laid back and spent more time laughing and joking. Another example of a positive personality change was seen in Patient 3534. At age seventy parts of the front of her brain were damaged when a tumour was removed. Her husband, who had been married to her for decades, described her as being stern, irritable and grumpy pre-surgery. Post-surgery she was happier, outgoing and chatty. Sadly, brain injuries do not always result in increases in happiness. They often turn once happy people into more troubled and aggressive beings.
Brain-imaging studies, where patients lie motionless in large machines which create 3D models of their brain, have allowed psychologists to identify parts of the brain that are active when a person thinks about a particular topic or event. Such work has revealed that groups of neurons in different parts of the brain are linked and are associated with positive and negative feelings, anxiety, despair and happiness. These scans have also revealed there are many differences between each of our brains, with no two brains being the same. We are each wired a little differently, with this wiring due to difference in our genes, our experiences and developmental noise.
Our brains, and to some extent our personalities, change with age, and this is because the way our brains work is not constant, as networks of neurons can be repurposed. Many of us will know children who were shy and reserved but who developed into outgoing adults. We may also know outgoing individuals who have become more introverted following a traumatic event. These changes occur because the role of neuronal networks in your brain is a little different from year to year. Some synapses linking neurons are reinforced by experience, while others fade if they are rarely used, with others being completely repurposed. Although we cannot predict how a particular network of neurons will result in a personality trait, we do know our brains are flexible, and that who we are changes from one month to the next as connections in our brains change as we experience the world around us.