6. Behavioral Genetics and Personality

We are approximately 99.9% genetically the same. This is because we have all faced the same evolutionary pressures. Evolutionary psychology focuses on the mechanisms we have in common. However, we are not completely similar—small differences among genes can result in quite a bit of variation. Scientists in the field of behavioral genetics also investigate human behavior, but from the perspective of individual differences. Much of it falls under what we know as personality. In this section we’re going to look at a little background and then crack open the fascinating concept of what it means to be you. Behavioral genetics is related to but not specifically part of evolutionary psychology and requires analysis for a comprehensive overview. 

To examine our differences scientifically, we need to understand the concept of heritability. It’s the statistic that estimates what proportion of phenotypic variance in a trait is due to genetic differences in a specific population at a given time. I’ll unpack this through an example. Let’s say hypothetically you were to provide ten children with an identical upbringing and education in every way. Then under identical conditions, you give them a test. You have controlled for (equalized) the environment, leaving only the possibility that some kids are innately smarter than others are. Therefore, the variance in the test results will solely be due to genetic differences. That’s heritability. 

The best way to study the heritability of any given trait is through twin and adoption studies. This allows you to equalize genetics by looking at pairs of genetically identical twins, specifically when they are raised apart (so the environment varies). If one twin receives a brilliant education and the other a mediocre one, you can then compare their test scores. If they both score high or low regardless of upbringing, then the heritability of intelligence must be high. If the environment has a big effect on scores, then heritability is low. If the heritability of intelligence is eighty percent, it means the differences are eighty percent due to genetic influence and twenty percent due to either the environment, prenatal environment, gene–environment interaction, random chance, or measurement errors within the study. 

These twin studies yield much interesting and sometimes counterintuitive information. As you age, heritability increases, meaning that genes have more influence on you, and the environment has less. In essence, you become yourself. What appears environmental is often actually genetic. For example, if a child raised in a violent household starts acting violently, we usually blame the upbringing. However, it has more to do with an inherited tendency for violence. The child’s twin will show a nearly identical propensity, even if raised in the most peaceful of households. Likewise, a violent upbringing is unlikely to encourage a child with no such genetic propensity to act violently. This is the same for most other traits. When the environment does affect behavior, it is more because of random factors (accidents, illnesses, unique individual exposures, random neurobiological processes) than the systematic ones (family environment, socioeconomic status, culture, education.) This is not to say that the environment doesn’t matter greatly in terms of life experience; rather, genes have a stronger influence than is sometimes presumed. 

Cognitive psychologist Steven Pinker’s description of genetic influence on psychological traits is the best I’ve heard. He said you must imagine that a gene is like gravity—it’s always there, and there’s nothing you can do to change its influence on you. However, you can temporarily counter it, for example by jumping in the air or building a plane. We’ve already learned that the DNA in genes contains instructions to build neural circuits that cause the propensities to act or feel in specific ways under certain input; you can temporarily counter them if it would be advantageous. Genes have a reaction range, meaning they allow for varying outcomes—to a certain extent. For example, if you train hard and eat healthily you can increase your running speed, but not indefinitely. Most traits are polygenic, or influenced by hundreds of different genes, and genes are pleiotropic, or have many different effects. Genetic recombination during meiosis is random to ensure genetic diversity in offspring. You may be like your parents, or you might not be at all. 

Some evolutionary psychologists believe that individual personality is simply noise—meaningless natural variation among already successful genes, like the exact size of your nose. Others claim that personality differences are intentional, a variety of specific strategies in the game of survival and reproductive success. Some strategies are frequency-dependent—what you do depends on what others are doing. If most people are being cautious and searching for food close to home, it may pay to be adventurous and wander a bit further out or vice versa. 

The best way to classify personality is through the Five-Factor Model, established through a statistical technique used to identify patterns in large sets of data, called factor analysis. Personality is broken down into five traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism. They lie on a continuum, like temperature, with a low end and a high end, but no distinct categories. The distribution of each trait in a population falls on a bell curve. For example, people with average agreeableness make up the substantial middle portion of the curve. It then slopes off on either side, with extremely disagreeable people on one end and extremely agreeable people on the other. The further out you get, the fewer there are. Evolution will only support strategies that have the potential of increasing fitness, so lets take a look at how this is possible. 

People who score high in openness are inventive, creative, willing to try new things, take greater risks, intellectually curious, and appreciate abstract ideas, whereas those low in openness are more practical, data-driven, closed-minded, and more likely to persevere. In individual animals you can differentiate between those who are driven to explore and those who prefer the safety of knowing their surroundings. At the far end of the bell curve, creativity turns into psychosis, which is why many artists have this proclivity. The safest bet is generally in the middle of the curve, but you can see how either end of the spectrum comes with survival and reproductive costs and benefits. We are designed to display our personality traits, for example, a tattoo can be a signal of openness. Evolutionary psychologist Geoffrey Miller believes the will to display can be just as strong as the will to power because they both come with significant fitness benefits. 

Those who score high in consciousness are efficient, organized, focused, reliable, prepared, punctual, self-disciplined, and good at impulse control, whereas those who score lower are flexible, spontaneous, extravagant, careless, and less reliable, prepared, or disciplined. In animals you most easily observe impulse control. It might appear that high conscientiousness is always better, but the time and energy such a personality requires can be costly. In an unpredictable environment it can be advantageous to seize the short-term rewards and forego saving for the future. Exercise is a classic display of conscientiousness, as are owning and taking care of pets. 

Extraverts are outgoing and energetic, energized by external activities and interactions, interested in breadth of experience, and are social, talkative, and enthusiastic. Introverts are more independent, reserved, solitary, internal, and more interested in depth of experience. One of the best predictors of happiness is extraversion. From a fitness standpoint, extraverts benefit from the resources associated with all the resulting relationships. Historically, introverts had quite a different advantage; they were far less likely to succumb to disease. In hunter-gatherer times, a stranger could carry an illness for which you had no immunity. If you don’t like going to crowded parties, ancient genes may be trying to keep you healthy. Even today, you become slightly more introverted if your immune system is weak. 

Individuals who score high on agreeableness are friendly, compassionate, helpful, trusting, kind, and interested in getting along, whereas their disagreeable counterparts are more critical, rational, skeptical, and self-interested. Men are on average more disagreeable than women are because they have more to gain by ascending the dominance hierarchy. 

Neuroticism (also referred to as emotional instability) is the trait most associated with suffering. Those high in neuroticism are reactive, sensitive, pessimistic, and more susceptible to stress, anxiety, and depression, whereas emotionally stable individuals are more resilient, confident, optimistic, calm, and less reactive or easily upset. It might be hard to see the upside of being highly neurotic, but stability is not only about affect—it’s also about amplification. Unstable people are passionate, and when things go well, they experience the highs more profoundly. They are often useful allies to have in your coalition. Women on average score slightly lower in stability than men do because it pays to be a little neurotic when looking after the offspring. 

Then there is also IQ, which is often grouped with personality but does not come with benefits at both ends of the spectrum. In fact, each additional IQ point is associated with one percent lower death rates because it serves as a protective force. There is a small correlation between IQ and openness, but for the most part, it’s independent from the five main traits. 

Because traits are genetically influenced and high in heritability, they usually do not change significantly by life events, although they do play a big role in how life events affect you. Think back to comparing genes to gravity—conscious and focused attention can periodically override the tendencies. However, in the words of Doug Lisle, life is too varied and nuanced to sustain this long term. (For example, if you’re disagreeable, there’s only so long you’re going to be able to pretend otherwise before something trips you up.) When you do observe what looks like a drastic shift in personality, it’s usually more of a behavioral outcome that resulted from a big shift in the cost-benefit analysis, and high underlying openness. (For example, a man who spent all his free time watching TV and eating junk food suddenly exercises religiously and watches his diet probably received some new information. Perhaps he suffered a heart attack, or an attractive woman moved in next door, and he was open enough to change.)

As we’re going to see in the next section, some curious things can happen when you get different combinations in the extremes of personality traits. Behavior and affect may look disordered but usually they are not. 

*The information from the section is primarily from Blueprint by Robert Plomin, Spent by Geoffrey Miller, and Personality by Daniel Nettle