10 Truths About Human Behavior
Human behaviors are hard to understand and behavioral geneticists spend a lot of their time trying to know what makes us tick and why. In the midst of a replication crisis, these ten truths have copious amounts of evidence to back them up. Instead of calling them laws or ‘rules of behavioral genetics,’ Robert Plomin refers to them modestly as findings. Others like Eric Turkheimer have called them laws. In his work, Turkheimer highlighted three findings that he believed were significant enough to be called laws of behavioral genetics. His initial laws comprise the first two of this list and law number 9. By 2016, when Robert Plomin published his paper on the most replicated findings in behavioral genetics, he had an expanded list of ten laws. Now let’s look at these laws and what they mean.
- All psychological traits show significant and substantial genetic influence
Human beings have evolved for millions of years. Many of the behaviors we have today were selected through evolutionary processes in our past. Human cognition and the brains we have also evolved significantly. It can be argued that all our psychological traits including cognitive abilities, personality, mental illnesses or psychopathology, and cognitive disabilities have significant genetic influences.
Scientists use the word heritability to describe genetic variance in populations; that is; differences within a population that can be attributed to genetic variance. This finding implies that genes influence all human behaviors including intelligence, psychopathology, and even personalities.
2. No traits are 100% heritable.
This law is a caveat to the first one. Just because genes influence human behaviors significantly does not mean these traits are 100% heritable. Genes may explain some variation in the population while the environment explains whatever remains. Our environments are important too when it comes to influencing human behavior. A good example is a trait like intelligence which has a heritability of between 60% and 80% in adults. Despite the high heritability, we are still left with 40% or 80% variance of human intelligence being influenced by the environment.
3. Heritability is caused by many genes of small effect
After scientists learnt that most behavioral traits had significant genetic influence, they rushed to look for the genes responsible. What they found is not what they expected. Instead of finding a “candidate gene” say for intelligence, what they found is thousands of genes that contributed towards intelligence. Even more revealing was the fact that none of these genes had significant ‘standalone’ influence on human behavior. All the genes that influence human behavior all tend to have smaller effects. Christopher Chabris and his colleagues relied on Genome wide Association Studies (GWAS) to show how many genes of smaller effects could impact behavior or psychological traits.
4. Phenotypic correlations between psychological traits show significant and substantial genetic mediation.
There are correlations between some human behaviors and traits. For example, there is a correlation between depression and anxiety. People who get depression are also more likely to get anxiety. This is what is meant by phenotypic correlations. As this rule suggests, phenotypic correlations such as depression and anxiety are mediated by genes, meaning there are some genes that predispose individuals to both depression and anxiety. These “generalist genes” also mediate the correlation between schizophrenia and bipolar, and the relationship between neuroticism and depression.
5. The heritability of intelligence increases throughout development
In early childhood, the heritability of intelligence is substantially lower than in adulthood. At age 9 the heritability of intelligence is 41%, which is lower compared to a 55% heritability in adolescence, and 66% heritability in early adulthood. Some studies have also found a heritability of 80% among adults. As previously highlighted, heritability is a measure of genetic variance within a population and does not imply people inherit 41% or 80% of their intelligence from parents. From this finding we realize that people become more intellectually similar to their parents at old age than they are in childhood. It also reveals that interventions to increase intelligence in adults are less likely to bear any fruits.
6. Age to age stability is mainly due to genetics
Age to age stability means the persistence of a trait or its continuity over time. This law seems obvious because genes are less likely to change across the life span. A person with a certain personality will continue having the same personality even as they age. If an individual has the genes that influence height, then he’ll get taller and taller over time. This effect is made possible by our genes. However, if you think about this law in terms of law number five, then it raises a conundrum. If intelligence is heritable, then why would its heritability increase with time? Some scientists have argued that the increase in the heritability of intelligence across the lifespan could be due to innovation. A process, whereby, new genes start influencing a trait over time.
The age to age stability of traits provides a different solution to that question. Genes are responsible for trait continuity, however, with time some genes could be amplified increasing the heritability of a trait. Instead of innovation, this solution suggests amplification. The age to age stability of traits, or the continuity of these traits is more evident in personality, intelligence, and psychopathology.
7. Most measures of the “environment” show significant genetic influence
In socialization research, scientists test the influence of the environment on human behavior by studying the environments in which these humans inhabit. If they want to learn more about the influence of the home environment on children, for example, they go to their homes, count the number of books in the shelves, interview parents, and observes the children playing at home. However, what had remained significantly hidden to most of these scientists is that human beings choose and modify their environments in ways that conform to their genetic propensities. An intelligent parent is pretty much likely to have books at home than a parent who is not very intelligent. As shown in the previous laws, traits like intelligence are influenced by genes. Therefore, counting books as a measure of environmental influences on children is a second tire measure of genetic influences. The same is true of human personality and psychopathology.
This is a very important law because it questions how much of the environmental influences on human behavior are actually environmental (Also see law 9). Behavioral geneticists rely on twins to test for genetic differences and heritability. However, socialization researchers only study the environment and have no way of telling apart what behavior is influenced by the environment and which one is influenced by gene-environment interactions.
8. Most associations between environmental measures and psychological traits are significantly mediated genetically
There are always correlations between environmental measures like socioeconomic status and cognitive traits. You’ve probably heard from the media someone saying children from rich families appear intelligent because they can afford good schools and study materials. These people tend to believe that increasing material conditions in society would make everybody intelligent. These people correlate the socioeconomic background of students to their intelligence. As usual, the correlation between parental socioeconomic status and a child’s intelligence is positive.
Another variant of that argument is that socioeconomic status determines academic performance and that children of wealthy backgrounds perform well in school because they are privileged in some ways. However, as law number eight suggests, correlations between environmental measures and psychological traits are mediated genetically. Typically, intelligent people are likely to become wealthy, and the correlation between their status and their children’s intelligence is mediated by the genetics that both share. The same is true of socioeconomic status and academic performance. Instead of concluding parental socioeconomic status is what causes intelligence in childhood, the more accurate conclusion is that genes mediate the correlation between parental socioeconomic status and their children’s intelligence or academic performance.
9. Most environmental effects are not shared by children growing up
The environment that most siblings share when growing up is the home environment. Brothers and sisters are often raised in the same home until they graduate high school or college. However, considering that a significant amount of human behavior can be explained by genetics, what remains of environmental influence? As law number 9 suggests, a significant amount of environmental influences on human behavior are not shared by children when growing up. That means the home environment does very little to influence the behavioral outcomes of children.
This law is powerful because it questions some fundamental assumptions of human society. These assumptions revolve around the influence of parenting on the behavioral outcomes of children. If the home environment does not influence human behavior significantly, then what does? This question is the subject of Judith Rich Harris’ book “The Nurture Assumption.” The book questions the fundamental assumption that the way parents rear their children has significant influence on their outcomes.
Middle Class parents may, for example, think that the way they parent their children, including reading them books, or the parenting styles as highlighted by Dina Baumrind, have significant influence on the child’s intelligence or behavioral outcomes. However, as this law suggests, siblings raised in the same family do not always have the same behavioral outcomes. Most similarities between them are genetic and do not reflect the influence of the home environment. This does not imply that parents are useless, instead, their contributions towards behavioral outcomes might be minimal. A good parent may give birth to a good child and vice versa. Parenting effects get overshadowed by genetic effects in this case. Harris is careful to separate genetic effects, gene and environment interactions, and other mediating effects in her theory of group socialization. I believe this is one of the most profound laws in behavioral genetics.