Genetics is the study of genes and the variation of characteristics that are influenced by genes—including physical and psychological characteristics. All human traits, from one's height to one's fear of heights, are driven by a complex interplay between the expression of inherited genes and feedback from the environment.
Scientists are tasked with a massive but increasingly plausible mission: mapping the pathway from one's genes to the person one sees in the mirror. What they learn about the power of genes has implications for understanding mental illness and psychological differences between individuals, as well as the psychological effects of non-genetic factors.
Genes help to define who an individual is inside and out. While non-genetic factors have a role to play, too, what scientists have learned about these influences can clash with common wisdom. A characteristic or behavior that appears to result from a child’s upbringing—such as a proneness to mental illness or divorce—may actually be largely a product of the genes she inherited from her parents. In fact, research investigating the influence of the family environment suggests that it accounts for a surprisingly small amount of the difference between people on characteristics that scientists measure.
A gene is the basic unit through which genetic information is stored and passed between generations. Physically, a gene is a specific section of one of the long, double-helix-shaped DNA molecules that appear in each cell of the body. Genes vary in size, comprising anywhere from hundreds to millions of the nucleotides that collectively make up DNA. Many (but not all) genes provide chemical “instructions” for the creation of protein molecules, or serve other roles that are integral to the function of an organism. Different versions of the same gene are called alleles.
The genome is the entirety of the genetic material contained in an individual. Human DNA is estimated to contain between 20,000 and 25,000 genes. The vast majority of each person’s genome is identical to that of the next person, but the portion that differs is consequential for how individuals develop.
A chromosome is a structure within a cell nucleus that is made up of a long DNA molecule and proteins that provide support. Each human cell contains 23 pairs of chromosomes, which together store a person’s genetic code.
In addition to visible traits like weight and eye color, psychological qualities such as personality traits (such as extraversion and agreeableness), intelligence, risk of mental illness, and many others are to some extent influenced by genetics. While genes do not account for all of the differences between people on these characteristics, research indicates that they have a substantial impact.
People’s levels of risk for all major psychiatric disorders are determined partly by genetics. Estimates of how much variability in risk can be attributed to genetic differences between individuals include: 75 percent or more for ADHD, autism spectrum disorders, bipolar disorder, and schizophrenia; 50 to 60 percent for alcohol dependence and anorexia nervosa, and 20-45 percent for anxiety disorders, OCD, PTSD, and major depressive disorder.
The pathway from genes to psychological dispositions and behavior is highly complex, but it runs through the brain: Genetic instructions influence brain development, and differences in the genetic code produce differences in how the brain is wired. These instructions do not completely determine how development plays out, leaving room for other factors (including chance events during development) in shaping behavior.
Genes may also influence a person less directly, through chains of cause-and-effect that involve the environment. For example, a genetically influenced trait (such as above-average extraversion) might lead someone to seek out situations (such as frequent social interactions) that reinforce that trait.
The phrase “nature vs. nurture” has been used as a shorthand for the question of how much a people’s inherent nature, or genetics, explain the differences between them, how much of these differences are instead explained by “nurture”—meaning upbringing, or people’s experience of their environments more generally.
While both genetics and environment play a part in shaping people’s characteristics, the phrase “nature vs. nurture” can be misleading. In many ways, genetics and experience interact (rather than working in opposition) to affect how a person turns out. And some portion of individual differences are due to neither nature nor nurture alone, but rather to inherent variability in the process of human development.
Chemical compounds can attach to genes, modifying their activity without changing the underlying DNA. These modifications are called epigenetic changes (and the study of them is epigenetics). Epigenetic changes happen normally as part of development, but they can also be influenced by environmental and experiential factors, such as diet and exposure to toxins. There is evidence suggesting that traumatic experiences may also lead to epigenetic changes that influence gene expression.
There is no one gene that activates a particular psychiatric disorder or any other complex psychological trait. In fact, many genes interact to influence the human brain. Normal and disordered psychological characteristics are polygenic, meaning that they are each shaped by a large number of genes. While rare mutations in certain genes may have a disproportionate impact, for the most part, each of the many relevant genetic differences plays a very small role in increasing or decreasing risk of a particular condition or influencing a given trait.
In medicine more broadly, there are certain genetic disorders that do primarily involve abnormalities in a single gene. These single-gene disorders include cystic fibrosis, Huntington disease, sickle-cell anemia, and Duchenne muscular dystrophy.
The X and Y chromosomes are also known as the sex chromosomes and play a fundamental role in determining the biological sex characteristics of an individual (such as reproductive organs). Females do not have Y chromosomes; they inherit one X chromosome from each parent. Males have an X chromosome, form their mother, and a Y chromosome from their father. The X and Y chromosomes also contain genes that affect traits not related to an individual’s sex.
Genealogy is the study of family lineage. While it is not a new practice, developments in DNA testing in recent decades brought a boom in enthusiasm about genealogy, with businesses using test results to provide clues about consumers’ genetic relatives and their geographic origins. There are a variety of reasons people might be drawn to genealogy, such as curiosity about a family’s deeper history among those who feel that they lack knowledge of or cultural ties to the past. One of the broader lessons of genealogy may be that far-flung people are more related than they might seem.
It may seem obvious that the genes people inherit from their parents and share with their siblings have an effect on behavior and temperament. Individuals are often noticeably more similar in a variety of ways to immediate family members than they are to more distant ones, or to non-relatives. Of course, there are plenty of notable differences within families as well. Scientists have employed an array of methods to drill down into how and to what extent genetic differences truly account for psychological differences.
Behavioral genetics, or behavior genetics, is the study of psychological differences between individuals and how genetic and non-genetic factors create those differences. Among other questions, behavioral genetics researchers have sought to determine the extent to which various specific differences in people’s behaviors and traits can be explained by differences in their genetic code.
Scientists use specialized methods to explore the links between genes and individual differences. Studies of twins who either do or do not have identical genomes allow for estimates of the degree to which genes drive the variation in psychological traits. Other methods, such as studying adopted children and their adoptive and biological parents, have been used as well. In recent years, genome-wide association studies (GWAS) have emerged as a major approach in behavioral genetics. A GWAS uses genetic testing to identify numerous genetic differences across many individuals, then analyze the association between these differences and personality traits or other outcomes.
Heritability is a measure of how much of the differences between people on a given characteristic can be attributed to genetics. More specifically, it is an estimate of the amount of variation between individuals in a given population that can be accounted for by genetic differences. Behavioral genetics research indicates that every trait is (at least) a little bit heritable—though the fact that a trait is heritable does not mean it is fixed.
Heritability estimates range from 0 to 1, or from zero percent to 100 percent. For example, if the heritability of a trait is estimated to be 50 percent, that suggests that about half of the overall variation between different people on measures of that trait—within the specific group of people measured—is due to differences in their DNA. (If heritability of a trait was 100 percent, identical twins, who share the same genetic code, would be exactly the same on that trait—but that doesn’t actually happen.)
Genes and the environment do not work completely independently. In what scientists call gene-environment interaction, aspects of the environment may have different effects on an individual depending on her genetic code. For example, adverse childhood experiences may have a severe impact on someone with a certain genetic disposition and a less-severe effect on someone with different genes. Gene-environment interaction can also work the other way around: the influence of genes on an outcome may depend on a person’s environment.
Given that genetics only accounts for a portion of the psychological differences between people, a genetic test will never be able to perfectly predict an individual’s behavior. Some researchers are hopeful that “polygenic scores”—which provide information about the likelihood of certain outcomes (such as developing a mental disorder) based on many small genetic variations—will serve as a useful tool for assessing risk in psychological domains. However, the predictive power of such scores is currently limited, and there are further limitations that lead other researchers to question how effective they will ultimately be for forecasting psychological outcomes.
Genome editing is a rapidly developing practice through which an organism’s genetic code is deliberately changed. Many of the genome-editing approaches in development today involve a mechanism called CRISPR/Cas9, which is adapted from a genome-editing system that appears in bacteria (as a defense against viruses). CRISPR enables the relatively quick and efficient targeting of specific genes in a cell. Scientists are pursuing CRISPR-based therapies for potential use in humans, with aims such as removing disease-causing genetic mutations. Genome editing technology could ultimately play a role in the treatment of mental illness.