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Individual Differences and Personality

The search for the magic number

I visited a day care center for preschoolers recently and was struck by how early the differences in children's traits and individual dispositions show up. Some children are more outgoing, while others are shy; some like to explore the environment and take risks, while others are more fearful; some get along well with others and some are bullies-even by age four. Young parents who have more than one child see immediate differences in the dispositions of siblings, as well as differences between their offspring and themselves.

At the other end of life, there are clear differences in how people age-some people simply seem to fare better than others. Even setting aside differences in physical health, and the various diseases that might overcome us late in life, some older adults live more dynamic, engaged, active, and fulfilling lives than others. Can you look at a five-year-old and tell whether they will be a successful eighty-five-year-old? Yes, you can.

The discovery that aging and health are related to personality was the result of a lot of work. First, scientists had to figure out how to measure and define personality. What is it? How do you observe it accurately and quantitatively? Here, they may have taken inspiration from Galileo, who said, "The job of the scientist is to measure what is measurable and to render measurable that which is not." And so they did.

Among the most solid findings is that a child's personality affects adult health outcomes later in life. Take, for example, a child who was always getting into trouble in elementary school and continued to do so as a preteen. As a teenager, they might have smoked cigarettes, drunk alcohol, and used marijuana. In personality terms, we might say that this teenager was sensation- and adventure-seeking, high on the quality of extraversion, low on conscientiousness and emotional stability. The kid would have been at increased risk for hard drug use, or being killed in a motor vehicle accident while driving drunk. If they survived these increased risks in young adulthood but didn't change their habits, they'd enter middle age with a highly inflated risk of lung cancer from smoking or liver damage from drinking. Even more subtle behaviors can influence outcomes many decades later: Early and compulsive exposure to the sun and sun tanning; poor dental hygiene; poor exercise habits; and obesity all take their toll.

One of the pioneers in the relationship between personality and aging is Sarah Hampson, a research scientist at the Oregon Research Institute. As Hampson notes, "Lack of self-control may result in behaviors that increase the probability of exposure to dangerous or traumatic situations and adversely affect health through long-lasting biological consequences of stress." She has found that childhood is a critical period for laying down patterns of behavior with biological effects that endure into adulthood. If you want to live a long and healthy life, it helps to have had the right upbringing. Childhood personality traits, assessed in elementary school, predict a person's lipid levels, blood glucose, and waist size forty years later. These three markers, in turn, predict risk for cardiovascular disease and diabetes. The same childhood traits even predict life span.

Although these correlations between early childhood and late adulthood personality are robust, they tell only a part of the story. People age differently, and part of that story has to do with the interaction of genetics, environment, and opportunity (or luck). Scientists developed a mathematical way of tracking personality, comparing traits as they differ across individuals or change within a person over time. With it, we can talk about age-related, culture-related, and medically induced changes in personality, such as occur with Alzheimer's disease. Often one of the first indications of a problem with your brain is a change in personality.

And in the past few years, developmental science has shown that people, even older adults, can meaningfully change-we do not have to live out a life that was paved for us by genetics, environment, and opportunity. The great psychologist William James wrote that personality was "set in plaster" by early adulthood, but fortunately he was wrong.

The idea that people retain the capacity to change throughout their life span didn't take hold until the mid-seventies, when an idea first put forward by psychologist Nancy Bayley was popularized by the German developmental psychologist Paul Baltes:

Most developmental researchers do accept the notion that developmental change is not restricted to any specific stage of the life-span and that, depending upon the function and the environmental context, behavior change can be pervasive and rapid at all ages. In fact . . . the rate of change is greatest in infancy and old age.

Not everyone takes advantage of this capacity, but it is there, like the ability to adjust your diet or your wardrobe. The events of your childhood can be overcome and transformed based on experiences you have later in life. Bayley and Baltes' big idea was that no single period of life holds supremacy over another.

Of course, the idea that people can change is the entire basis of modern psychotherapy. People seek psychiatrists and psychologists because they want to change, and modern psychiatry and psychology are largely effective in treating or curing a great number of mental disorders and stressors, especially phobias, anxiety, stress disorders, relationship problems, and mild to moderate depression. Some of these volitional changes revolve around improved lifestyle choices, while others entail changing our personalities, sometimes only slightly, to give us the best chance of aging well. To implement the changes that will be most effective, each of us might think about the fundamental components of how we are now, how we used to be, and how we'd like to be.

The collection of dispositions and traits that we have in any given period comprise our personalities. All cultures tend to describe people using trait-based labels, such as generous, interesting, and reliable (on the positive side) or stingy, boring, and erratic (on the negative side), along with more or less neutral or context-dependent terms such as boyish and breezy. This "trait" approach, however, can obscure two important facts: (1) we often display different traits as situations change, and (2) we can change our traits.

Few people are generous, interesting, or reliable all the time-opportunity and the fluidly evolving situations in which we find ourselves can exert a strong pull on what may be genetic predispositions toward certain behaviors and certain habitual ways of presenting ourselves to the world. Traits are probabilistic descriptions of behavior. Someone who is described as high on one trait (having a lot of it) will display that trait more often and more intensely than someone low on that trait. Someone who is agreeable has a greater probability of displaying agreeableness than someone who is disagreeable, but disagreeable people are still agreeable some of the time, just as introverts are extraverted some of the time.

Culture plays a role as well, both macro- and microculture. What is considered shy, reserved behavior in the United States (macrolevel culture) might be regarded as perfectly normal in Japan. And staying within the United States for the moment (microlevel culture), behavior that is considered acceptable in a hockey game might not be acceptable in the boardroom.

Booker T. Washington wrote that "character, not circumstance," makes the person. Ralph Waldo Emerson wrote, "No change of circumstances can repair a defect of character." While character makes for a good story or poem, in reality we are less shaped by character traits than we think, and more than we realize by the circumstances that life deals us-and our responses to those circumstances. It would be nice to be able to grade these circumstances from severely deleterious to benign, but what makes that impossible to do is individual differences in the way we respond to things. Some children who were (or felt) abandoned by their parents grow up to be well-adjusted, do-gooding members of society; others become axe murderers. Resilience, grit, and gratitude for the small things in life ("at least I still have food to eat") are personality traits that are unevenly distributed in the population.

We think of our genes as influencing physical traits, like hair color, skin color, and height. But genes also influence mental and personality traits, such as self-assuredness, a tendency toward compassion, and how emotionally variable we are. Look at a room full of one-year-olds and it is apparent that some are more calm than others, some more independent, some loud, some quiet. Parents with more than one child marvel at how different their personalities were from the start. I carefully referred to genes influencing traits because the effect of genes is not chiseled in stone. Your genes don't dictate how you'll be, but they do provide a set of constraints, limits on how your personality will be shaped. Genetics is not an edict-the traits that our genes contribute to still need to navigate the twisty and unpredictable roads of culture and opportunity. Complex traits are best described as emergent properties that you cannot read in any one gene, nor even in a large set of genes, because how the genes express themselves over time is critical to the development of the trait as a social reality.

Genes can be present in your body but in a dormant state, waiting for the right environmental trigger to activate them-what is called gene expression. A traumatic experience, a good or bad diet, how and when you sleep, or contact with an inspiring role model can cause chemical modifications to your genes that in turn cause them to wake up and become activated, or to go to sleep and turn off. The way the brain wires itself up, both in the womb and throughout the life span, is a complex tango between genetic possibilities and environmental factors. Neurons become connected whenever you learn something, but this is subject to genetic constraints. If you've inherited genes that contribute to making you five feet tall, no amount of learning is likely to get you into the NBA (although Spud Webb is five foot seven and Muggsy Bogues is five foot three). More subtly, if your genes constrain the auditory memory circuits in your brain-perhaps because they favor visual-spatial cognition-you're unlikely to become a superstar musician no matter how many lessons you take, because musicianship relies on auditory memory.

One way to think about gene expression is to think of your life as a film or multiyear TV series. Think of your DNA as the script: the set of instructions, dialogue, and stage directions for all the participants in the film. Your cells are the actors. Gene expression is the way that the actors decide to express that script. The actors may bring a certain interpretation to those words, based on their experience, and might surprise even the writers.

And, of course, the actors interact with and play off one another, for better or for worse. Jason Alexander, the actor who played George Costanza on Seinfeld, complained about how difficult it was to work with Heidi Swedberg (who played George's fiancŽe, Susan). "I couldn't figure out how to play off of her. . . . Her instincts for doing a scene, where the comedy was, and mine were always misfiring." Julia Louis-Dreyfus and Jerry Seinfeld had similar complaints and reportedly said that doing scenes with her was "impossible." But the chemistry between Alexander, Louis-Dreyfus, Seinfeld, and Michael Richards (Cosmo Kramer) was palpable, making Seinfeld the most successful comedy series in history.

Your genes, then, give you a kind of life script with only the most general things sketched out. And from there, you can improvise. Culture affects the ways you interpret that script, as do opportunity and circumstance. And then, once you interpret the script, it influences the way others respond to you. Those responses in your social world can change your brain's wiring and chemistry, in turn affecting how you'll respond to future events and which genes turn on and off-over and over again, cascading in complexity.

The second feature in the triad, culture, plays an important role in our understanding of traits. Humility is more valued in Mexico than in the United States, and more valued in rural Wisconsin than on Wall Street. Polite in Tel Aviv might be thought of as rude in Ottawa. The terms we use to describe others are not absolutes; they are culturally relative-when we describe differences in personality traits, we're necessarily talking about how an individual compares to their society and to their societal norms.

Family is a microculture, and traditions, outlook, political and social views differ widely, especially within large industrialized countries. Go door to door in any town or city and you'll find a wide range of attitudes about things as mundane as whether friends can just drop by or need to schedule in advance; how often teeth should be flossed (if at all); or whether TV and device time are regulated. And these unique family cultural values map onto particular personality traits: spontaneity, conscientiousness, and willingness (or at least ability) to follow rules. Culture is a potent factor in who we become.

The third part of the developmental triad is opportunity. Opportunity and circumstance play a larger part in behavior than most of us appreciate, and they do this in two different ways: how the world treats us, and the situations we find (or put) ourselves in.

Fair-skinned children burn more quickly in the sun than dark-skinned children and so may spend less time outdoors; skinny children can explore the insides of drainage pipes and the tops of trees more easily than heavy children. You may start out with an adventure-seeking personality, but if your body won't let you realize it, you may seek other experiences, or adventure in less physical ways (like video games-or math).

Apart from these physical features, we all play roles, in our families and in society. The eldest child in a multi-child household tends to take on some of the parenting and instruction of the younger ones; the youngest child may be relatively coddled or ignored, depending on the parents; the middle child may find herself thrust into the role of peacemaker. These factors influence our development, but again, as with genes, they are not deterministic-we can break free of them to improvise, to create our own futures, but it takes some effort (and for some, a lot of false starts, failures, and therapy).