Are kings, generals, and ministers merely born into their kind?
Chen Seng, asked rhetorically during the Chen Sheng Wu Guang uprising in 209 BC.
Let us then suppose the mind to be, as we say, white paper void of all characters, without any ideas. How comes it to be furnished? Whence comes it by that vast store which the busy and boundless fancy of man has painted on it with an almost endless variety? Whence has it all the materials and knowledge? To this I answer, in one word, from EXPERIENCE.
John Locke, An Essay Concerning Human Understanding
What is more important, nature or nurture? This question, popularized by the English polymath Francis Galton (also a proponent of eugenics, and scientific racism), still fascinates us. Some people believe that human nature is infinitely malleable; that our social and cultural environment determines our behavior. Without culture, many believe, we would be aimless. The great American anthropologist Clifford Geertz, argued that “Undirected by cultural patterns – organized systems of significant symbols – man’s behavior would be virtually ungovernable, a mere chaos of pointless acts and exploding emotions, his experience virtually shapeless.”[1]
In this view, it is the cultural milieu that determines if people will conflict with each other or will live in harmony and happiness. Supporters of this view believe that the mind is a kind of tabula rasa or blank slate, free of content until written on by the hand of experience and learning (see Locke’s quote above). The most beautiful and the ugliest inclinations of humans are not part of a fixed and biologically given human nature but result from the social process which creates humans. That is, human nature is shaped by the surrounding cultural and social environment. The doctrine of the blank slate, as this is known, became entrenched in intellectual life in a form that has been called the Standard Social Science Model.
On the other hand, genetic determinists argue that behaviors such as criminality can be explained by the action of genes alone. As we have seen, there is evidence that supports a constant and predictable human nature (Idea 5). Humans develop personalities and dispositions that are stable throughout their lives. Some are born introverts and although they may be able to manage and control their emotions and behavior to appear otherwise, they stay that way for the rest of their lives. The same happens to extroverts.
In addition, we are all born with ontological assumptions regarding how the world works. For example, we all make use of evolved psychological mechanisms in our decision-making such as rules of thumb or heuristics. These psychological qualities are among the best-preserved aspects of human nature (see Idea 10).
Despite the impressive advances in genetics and psychology in recent years, there is still much misunderstanding in our society regarding the mutability of human nature. Much of this is rooted in ideology, though. The idealists adhere to the blank slate thesis because the immutability of human nature means their utopian dreams are destined to fail in the face of a man that is not perfectible. The staunch conservative worries that inherited traits will absolve criminals of moral responsibility with the argument that the “genes made me do it.”
While most scientists reject the idea that biology is destiny and that the mind is a blank slate, many of them are uncomfortable with any characterization of a human trait as inborn or innate. Instead, there is a widespread assumption that human behavior is the “inextricable interaction” of nature and nurture. The Harvard cognitive psychologist Steven Pinker summarizes this view as follows: [2]
No one today believes that the mind is a blank slate; to refute such a belief is to tip over a straw man. All behavior is the product of an inextricable interaction between heredity and environment during development, so the answer to all nature-nurture question is “some of each.” If people only recognized this truism, the political recrimination could be avoided. Moreover, modern biology has made the very distinction between nature and nurture obsolete. Since a given set of genes can have different effects in different environments, there may always be an environment in which a supposed effect of the genes can be reversed of cancelled; therefore, the genes impose no significant constraints on behavior. Indeed, genes are expressed in response to environmental signals, so it is meaningless to try to distinguish genes and environments; doing so only gets in the way of productive research.
Box 13: Genes and Behavior: Why People Become Addicted and Hostile To what extent genes shape human behavior is a controversial topic, and probably will remain so for years to come. Nonetheless, we are beginning to understand how genes can influence complex behaviors such as addiction and hostility. Most people addicted to alcohol and nicotine have a higher tolerance to these substances than the average population. Nicotine, for example, is a natural insecticide found only in tobacco. Like other insecticides, it is toxic to humans. Researchers have found that people with certain gene variations may be able to tolerate more nicotine before they get sick and, as a result, smoke more in the first place, promoting addiction. These gene variations, known as polymorphisms, represent alterations to a gene’s DNA sequence and occur naturally in the population. The same variations are also associated with alcohol consumption, which means that probably common genes play a part in why there is a strong link between nicotine and alcohol intake. Of course, to become addicted to alcohol and nicotine we must try them out first, so restricting access to cigarettes and alcohol for the young is a good way to start. On the other hand, the type of aggressive behavior we think of as psychopathic may have some genetic basis that involves abnormally low levels of the neurotransmitter serotonin. The tainted history of using biology to explain criminal behavior has pushed criminologists to reject or ignore genetics and concentrate on social causes: poverty, corrosive addictions, and guns. Now that the human genome has been sequenced, and scientists are studying the genetics of areas as varied as alcoholism and party affiliation, criminologists are cautiously returning to the subject. Once again, gene polymorphisms appear to influence individual variance. Apparently, hostile men (males are responsible for most violent crimes) release high levels of cortisol, a key stress-hormone. The researchers speculate that, after being deprived of serotonin, these men’s neural receptors are exquisitely sensitive and primed to over-react, in part by producing extra cortisol. Studies have also shown that a variant of a gene located on the X chromosome called monoamine oxidase A, or MAOA, is associated with violence and aggression. One study, for example, found that boys who carry a low-activity variant of this gene, MAOA-L, were more likely to join gangs and those who did were four times more likely to use weapons in a fight. While I do not believe that there is such thing as a “crime gene”, nor that “the genes made me do it” is a valid excuse, it is important to understand the genetic influences in people’s addictive and aggressive behavior. This can inform us about what particular aspects of the environment we should examine. See Butt C. M. et al. (2005), Modulation of nicotine but not ethanol preference by the mouse Chrna4 A529T polymorphism, Behavioral Neuroscience, 119,1,26-37; Henning J. et al (2005). Two types of aggression are differentially related to Serotonergic activity and the A779C TPH polymorphism, Behavioral Neuroscience, 119,1,16-25; Beaver, K. et al. (2010). Monoamine oxidase A genotype is associated with gang membership and weapon use, Comprehensive Psychiatry, 51, 2, 130-134.
The attitude that disentangling the nature-nurture interactions is not only impossible but also meaningless is referred to as “holistic interactionism” by Pinker. For example, the Stanford biologists Paul Ehrlich and Marcus Feldman write “With expressions such as ‘genes are responsible for 50 percent of,’ or ‘genes contribute 50 percent of,’ a behavior, this language gives the impression that genetic and environmental contributions to human behaviors are separable. They are not.” [3] Nonetheless, Pinker believes this approach is counterproductive and can stand in the way of achieving a true understanding of human nature and behavior.
The role of nature, nurture, and chance in human development
Is it possible to disentangle the nature and nurture interactions as Pinker claims? To be sure, there are situations that can be readily labeled as “environmental” or “genetic.” The fact that we speak English or Spanish is 100 percent a function of the environment we are born into; there is no such thing as “English language” genes or “Spanish language” genes. In contrast, we know that conditions like autism and schizophrenia are highly heritable (see Box 14).
What about intelligence? For many decades, psychologists have looked for the causes of individual differences in cognitive ability (as measured by IQ tests). The conventional wisdom has been that IQ is strongly influenced by parenting practices and role models. This belief is flawed, however, as it was based on studies that compare parents and children but did not control for genetics. Researchers have tried to correct these flaws with studies of twins and adoptees and have discovered that, in fact, all behavioral and cognitive traits are partly (though never completely) heritable.[4] That is, some of the variation among individual IQs must be attributed to differences in their genes (see Idea 6).
The conclusion follows from repeated discoveries that identical twins reared apart (who share their genes but not their environment) are highly similar; that ordinary identical twins (who share their environment and all their genes) are more similar than fraternal twins (who share their environment but only half of their genes); and that biological siblings (who share their environment and half their variable genes) are more similar than adoptive siblings (who share their environment but not their genes). These studies have been replicated in large samples from several countries and have ruled out the most common alternative explanations (such as selective placement of identical twins in similar adoptive homes).
One must be careful in generalizing these results as there may be “hidden” correlations between genes and environments that are unaccounted for. For example, the estimated heritability of IQ on individuals from advantaged backgrounds is higher than in those from disadvantaged backgrounds. That is because better environments allow more variance in IQ to be expressed: potential geniuses have trouble developing into Einsteins in areas without schools and libraries.[5]
Another important finding from the nature-nurture studies is the extent that chance plays in human development. The evidence shows that there are variations in personality that cannot be accounted for by genes, family environment or peer influence. For example, identical twins growing in the same home share their genes, their parents, their siblings, and their culture. Yet, correlations in their traits (e.g., personality, intelligence) are about 50%. Peer influence cannot account for the differences because identical twins largely share their peer groups. This shows that human development does not follow a pre-specified blueprint and that the role of sheer chance is significant (factors such as prenatal blood supply, exposure to toxins, pathogens, hormones, stress, etc. have all been studied).
Box 14: Heritability Heritability is an important but often misunderstood concept. In a nutshell, it is a statistic that estimates the degree of variation in a phenotypic trait (an observable and measurable trait, e.g., height) in a population that is due to genetic variation between individuals in that population. It measures how much of the variation of a trait can be attributed to variation of genetic factors, as opposed to variation of environmental factors. As we saw, the traits of an individual are always a complex interweaving of both. The concept of heritability can then be expressed in the form of the following question: “What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?” One easy-to-understand way of measuring heritability is through one-generation selection hypothetical experiment. Individuals with extreme values of a trait are bred to one another, for example, the heaviest individuals from a population. The offspring are then raised in the same environment, and their average weight calculated. If the average weight of the offspring does not increase over that of the entire population (not just of the heavy parents) in the previous generation, the heritability is zero. On the other hand, if the average weight of the offspring equals that of the heavy parents, the heritability is 100%. Thus, a heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. The study of identical twins has also been useful in determining the heritability in humans of a particular trait. In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. In many cases, it has been found that genes make a substantial contribution, including psychological traits such as intelligence and personality. Yet heritability may differ in other circumstances, for instance environmental deprivation. Examples of low, medium, and high heritability traits include:
See, e.g., Polderman T. J. C. et al. (2015). Meta-Analysis of the Heritability of Human Traits based on Fifty Years of Twin Studies. Nature Genetics, 2015 Jul;47(7). Also, Ehrlich, P. & Feldman, M. (2007). Genes, environments & behaviors, Dædalus, Spring.
The uterine environment seems to have a long-term impact on developing fetuses. Over the past two decades, scientists have been developing a new understanding of our prenatal experiences and how they exert lasting effects on us from infancy well into adulthood. Studies have shown that a single exposure to an environmental toxin may produce damage that is passed on to multiple generations, and that conditions as varied as diabetes, heart disease, and mental illness may start in the womb. For example, there is a well-established link between low birth weight and later cardiovascular disease.[6]
The study of the possible impact that the environment can have on multiple generations is still in its early phases. One obvious problem is that it takes a long time to establish a connection between cause and effect. Nonetheless, some clues are emerging as to how these mechanisms may operate in practice. As we saw in Idea 2, natural selection operates through mechanisms like mutation and genetic drift, which alter the genome and thus the phenotype, or appearance of the organism. We also saw that the effects of genetic variation are only apparent in specific environments, which begs the question of what exactly is that genes do. Though genetic variation provides “potential” for influencing the development of the individual, this potential may only be apparent when environmental triggers are present.
To better understand these mechanisms, I use an analogy put forth by Frances Champagne, a Columbia University neurobiologist.[7] She sees genes, which are sequences of DNA, as a vast library of books that have been ordered and arranged very precisely by a meticulous librarian. Each one of these books contains a wealth of knowledge that has the potential to inspire whoever should choose to read it. Once read, the information in those books can have limitless consequences and can perhaps even lead to the reading of more books, but that refers to the book's potential. However, many of these books contain information of such an obscure nature that they will seldom be removed from their position within the rows upon rows of books around them. Even books containing valuable information may sit undisturbed and unread, gradually collecting dust over the years. This may be particularly true if the book is hard to get to. It may be located on a shelf that is particularly difficult to reach or blocked by some piece of furniture that has been moved to create more space elsewhere.
Just like the books in the library, our DNA “sits” in our cells waiting to be read. The reading or “expression” of DNA can, like the books in our library, have limitless consequences. However, without the active process that triggers “expression,” this potential may never be realized. What exactly triggers “expression”? The environment around the DNA seems to contain those critical factors that make it possible to read the DNA and, in essence, make it do something. Conversely, there are factors that make the DNA less accessible and thus less likely to be “expressed.” In fact, DNA’s “expression” can be easily blocked by factors in our cells that make the DNA harder to get to and more difficult to read.
The power of the environment to shape behavior
There is growing evidence that the processes that change the potential of a gene being read are environmentally driven. Several studies support the notion that the nutritional and social environment, as well as the processes of learning and memory, can shape the likelihood that genes will be read. In some cases, genes can be entirely “shut off” through these processes. “Just as is the case for our library of books,” writes Champagne, “the gene is there, but it sits unread collecting dust.”[8]
The factors that give context to DNA and determine the accessibility of DNA are referred to as “epigenetic” meaning “in addition to genetic.” Epigenetics investigates alternative mechanisms of gene expression that do not imply changes in the underlying DNA sequence. The study of epigenetic factors has become an active, and promising, area of research in the last few years. It has helped to better understand the complex heritability process. In general, heritability is calculated by comparing the stability of a trait in identical (monozygotic) twins and fraternal (dizygotic) twins, with an increase in monozygotic concordance in contrast to dizygotic twins indicating increased heritability.
Recent evidence suggests that some aspects of the epigenetic characteristics of a cell are heritable. While epigenetic changes do not alter the DNA, they leave chemical “marks” on genes that dictate how active they are, not just in the organism but in later generations. For example, if male mice suffered from social withdrawal, because of maltreatment in the first weeks of life, so too did their pups and grand pups, even if they were nourished and treated well from birth. Remarkably, they bore the same chemical markers on their DNA as their maltreated forebears. The evidence is accumulating that mental health problems can be “passed down” in this way, and that exposure to prenatal environmental cues (e.g., poor diet during pregnancy, recreational drugs) can alter the chemical markings on genes that are linked with mental disorders such as manic depression and schizophrenia. What remains to be known, is for how many generations of “molecular memory” of epigenetic marks can persist.[9]
Going back to our analogy, we can think of an inheritance of the rows of books, but also of the shelves, furniture, and particular ordering of those books that leave some volumes readily accessible, and others hidden in obscurity. Identical twins share the same library, whereas fraternal twins may have different libraries, containing a few different books but also potentially having a completely different architecture. The shelves, furniture, and layout of a library can be changed dynamically. Despite having the same books, a library can undergo dramatic changes. At a molecular level, those changes to the “epigenetic” characteristics of a cell can be induced by the environment and alter the characteristics of the cells that go on to create offspring. “Thus,” writes Champagne, “we inherit far more from our parents than just DNA. The environments to which they are exposed may lead to heritable changes that alter the development of the next generation; a molecular reality that would make any Lamarckian quite proud.”[10]
To conclude….
The findings discussed in this idea take us beyond the nature nurture debate to a deeper understanding of how to conceptualize interactions between genes and environments. They do not necessarily invalidate Pinker’s goal of separating the influence of the genes from those of the environment, but they add a new kink to the already convoluted relationship between nature and nurture. Could it be, for example, that extreme poverty leaves “markers” that condemn later generations to a life of hardship and underachievement? These are critical questions that will need to be addressed by scientists in years to come. The fact is genes and environment are constantly interacting and it would be impossible to consider one without the other. I will discuss some of these important issues further in Idea 16.
[1] Clifford Geertz (1973), The interpretation of cultures. New York: Basic Books, p. 49. [2]Pinker, S. (2004). Why nature & nurture won’t go away, Dædalus, Fall: p.7. [3]Ehrlich, P. & Feldman, M. W. (2007). Genes, environments & behaviors, Dædalus, Spring: p.6. [4]Bouchard, T. J. (1998). Genetic and environmental influences on intelligence and special mental abilities, Human Biology, 70, p.257-259. [5] Turkheimer, S. E. et al. (2003). Socioeconomic status modifies heritability of IQ in young children, Psychological Science, 14, p.623-628. [6]See, e.g., Paul, A. M. (2010). Origins: How the Nine Months Before Birth Shape the Rest of Our Lives, Free Press. [7] Champagne, F. A. (2009). Beyond nature vs. nurture: Philosophical insights from molecular biology, Observer: Association for Psychological Science, 22, 4 (April 2009). [8] Ibid. p.2. [9] See, e.g., Franklin, T. et al. (2010). Epigenetic transmission of the impact of early stress across generations, Biol Psychiatry, 68(5).p.408-415. A non-technical description of epigenetics can be found in “Sins of the fathers,” by Andy Coghlan, New Scientist, Nov 6-12, 2010. [10]Champagne, F. A. (2009). Beyond nature vs. nurture: Philosophical insights from molecular biology, Observer: Association for Psychological Science, 22, 4 (April 2009). p.3.
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