14.5 Genetic and Environmental Influences on Personality

One question that is exceedingly important for the study of personality concerns the extent to which it is the result of nature or nurture. If nature is more important, then our personalities will form early in our lives and may be difficult to change later. If nurture is more important, however, then our experiences are likely to be particularly important, and our personalities may change in response to experiences over time. While identical twins Paula Bernstein and Elyse Schein turned out to be very similar even though they had been raised separately, those traits that they share are likely to be the result of genes, but environments, particularly those that are unique to individuals, are important in shaping personality. In this section, we will see that personality traits seem to be determined by both genes and environments.

In the nucleus of each cell in your body are 23 pairs of chromosomes. One of each pair comes from your father, and the other comes from your mother. The chromosomes are made up of strands of the molecule deoxyribonucleic acid (DNA), and the DNA is grouped into segments known as genes. A gene is the basic biological unit that transmits characteristics from one generation to the next. Human cells have about 25,000 genes. The genes of different members of the same species are almost identical. The DNA in your genes, for instance, is about 99.9% the same as the DNA in my genes and in the DNA of every other human being. These common genetic structures lead members of the same species to be born with a variety of behaviours that come naturally to them and that define the characteristics of the species. These abilities and characteristics are known as instincts, which are complex inborn patterns of behaviours that help ensure survival and reproduction (Tinbergen, 1951). Different animals have different instincts. Birds naturally build nests, dogs are naturally loyal to their pack, and humans instinctively learn to walk, speak, and understand language.

However, the strength of different traits and behaviours also varies within species. Rabbits are naturally fearful, but some are more fearful than others; some dogs are more loyal than others; and some humans learn to speak and write better than others do. These differences are determined in part by the small amount — in humans, the 0.1% — of the differences in genes among the members of the species.

Personality is not determined by any single gene, but rather by the actions of many genes working together. There is no “IQ gene” that determines intelligence, and there is no “good marriage-partner gene” that makes a person a particularly good marriage bet. Furthermore, even working together, genes are not so powerful that they can control or create our personality. Some genes tend to increase a given characteristic, and others work to decrease that same characteristic. The complex relationship among the various genes, as well as a variety of random factors, produces the final outcome. Furthermore, genetic factors always work with environmental factors to create personality. Having a given pattern of genes does not necessarily mean that a particular trait will develop because some traits might occur only in some environments. For example, a person may have a genetic variant that is known to increase their risk for developing emphysema from smoking, but if that person never smokes, then emphysema most likely will not develop.

Studying personality using behavioural genetics

Perhaps the most direct way to study the role of genetics in personality is to selectively breed animals for the trait of interest. In this approach, the scientist chooses the animals that most strongly express the personality characteristics of interest and breeds these animals with each other. If the selective breeding creates offspring with even stronger traits, then we can assume that the trait has genetic origins. In this manner, dog breeders have bred dogs to be friendly (e.g., labrador retrievers) or protective (e.g., rottweilers).

Although selective breeding studies can be informative, they are clearly not useful for studying humans. Obviously, we cannot do experiments on people to see what genes or environments produce specific personality traits. To understand the relative influence of genes and environments on human traits, psychologists rely on behavioural genetics — a variety of research techniques that scientists use to learn about the genetic and environmental influences on human behaviour by comparing the traits of biologically and nonbiologically related family members (Baker, 2004). Behavioural genetics is based on the results of family studies, twin studies, and adoptive studies.

A family study starts with one person who has a trait of interest — for instance, a personality trait such as extraversion — and examines the individual’s family tree to determine the extent to which other members of the family also have the trait. The presence of the trait in first-degree relatives (e.g., parents, siblings, and children) is compared with the prevalence of the trait in second-degree relatives (e.g., aunts, uncles, grandchildren, grandparents, and nephews or nieces) and in more distant family members. The scientists then analyze the patterns of the trait in the family members to see the extent to which it is shared by closer and more distant relatives.

Although family studies can reveal whether a trait runs in a family, it cannot explain why. In a twin study, researchers study the personality characteristics of twins. Twin studies rely on the fact that identical (i.e., monozygotic) twins have essentially the same set of genes, while fraternal (i.e., dizygotic) twins share, on average 50% of their genes (half of the half from each parents – as in any set of full siblings). The idea is that if the twins are raised in the same household, then the twins will be influenced by their environments to an equal degree, and this influence will be pretty much equal for identical and fraternal twins. In other words, if environmental factors are the same, then the only factor that can make identical twins more similar than fraternal twins is their greater genetic similarity.

In a twin study, the data from many pairs of twins are collected, and the rates of similarity for identical and fraternal pairs are compared. If the correlation between degree of extraversion in identical twins were higher than the correlation for fraternal twins, it would be reasonable to argue that extraversion has some genetic influence.

However, we must also account for the environments of those being studied. In the example above, it is possible that the environments of identical and fraternal twins are different in some ways that relate to extraversion; furthermore, individual twins living in the same household as their twin may experience unique environmental influences. Within a family, some environments are shared (e.g., having the same parents); however, some are nonshared (e.g., friends, activities, or even treatment by a parent). Shared environments are likely to exert similar effects on family members, whereas nonshared environments are likely to result in differences between family members. Thus, any study of personality needs to account for genes, shared environmental effects, and nonshared environmental effects. In the typical twin study, all three sources of influence are operating simultaneously, and it is possible to determine the relative importance of each type. One key finding about environments is that, while parents do influence children’s development of personality and behaviour in early childhood, shared environments (e.g., having the same parents) seem to have little effect on personality differences in adulthood (Roberts & DelVecchio, 2000). Instead, the nonshared environments are more important in influencing personality differences.

Twin studies are complemented by adoption studies, which compare biologically related people, including twins, who have been reared either separately or apart. Nowadays, twins are less likely to be adopted by two different families, but past adoption practices have resulted in natural experiments, such as the example of Paula Bernstein and Elyse Schein discussed earlier. Evidence for genetic influence on a trait is found when children who have been adopted show traits that are more similar to those of their biological parents than to those of their adoptive parents. Evidence for environmental influence is found when the adoptee is more like their adoptive parents than their biological parents.

According to Thomas Bouchard, David Lykken, Matthew McGue, Nancy Segal, and Auke Tellegen (1990), twin and adoption studies show that approximately 40 to 50% of the individual differences in personality traits are the result of genetic differences between people, while the other 50 to 60% are the result of environmental differences. Thus, it is fair to say that your personality — in comparison to your siblings, for example — differs in part because of genetic differences and in part because of unique environmental experiences. In other words, both nature and nurture combine.

The major influence on personality is nonshared environmental influences, which include all the things that occur to us that make us unique individuals. These differences include variability in brain structure, nutrition, education, upbringing, and even interactions among the genes themselves. The genetic differences that exist at birth may be either amplified or diminished over time through environmental factors. The brains and bodies of identical twins are not exactly the same, and they become even more different as they grow up. As a result, even genetically identical twins have distinct personalities, resulting in large part from environmental effects.

Because these nonshared environmental differences are nonsystematic and largely accidental or random, it will be difficult to ever determine exactly what will happen to a child as they grow up. Although we do inherit our genes, we do not inherit personality in any fixed sense. The effect of our genes on our personality is entirely dependent on the context of our life as it unfolds day to day. Based on your genes, no one can say what kind of human being you will turn out to be or what you will do in life.

References

Baker, C. (2004). Behavioral genetics: An introduction to how genes and environments interact through development to shape differences in mood, personality, and intelligence. Washington, DC: American Association for the Advancement of Science.

Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota study of twins reared apart. Science, 250(4978), 223–228.

Roberts, B. W., & DelVecchio, W. F. (2000). The rank-order consistency of personality traits from childhood to old age: A quantitative review of longitudinal studies. Psychological Bulletin, 126(1), 3–25.

Tinbergen, N. (1951). The study of instinct (1st ed.). Oxford, England: Clarendon Press.