The Nature-Nurture Question Notes
The Nature-Nurture Question
Eric Turkheimer explores the "nature-nurture question," a fundamental inquiry into the origins of behavior. It examines the extent to which our traits are influenced by genetics (nature) versus environment and experiences (nurture). Behavior genetics uses methods such as family studies and DNA analysis to empirically study these influences. The field grapples with complex conceptual issues, revealing that nature and nurture always interact. The central challenge is understanding how they combine to shape specific behaviors.
Learning Objectives
Understand the nature–nurture debate and its enduring fascination.
Recognize the empirical challenges in studying nature–nurture questions.
Know the primary research designs used to investigate these questions.
Appreciate the complexities of nature–nurture and the lack of simple answers to seemingly simple questions.
Introduction
The nature-nurture problem is one of three problems that is at the intersection of philosophy and science. It is related to the mind-body problem and the free will problem. The controversy and offense caused by nature–nurture stems from its impact on our sense of moral character. Height, for example, is seen as a genetic gift, whereas dedication is praised. However, most characteristics are not clear-cut, as even a violinist's skill may involve some innate qualities. The origin of personal traits like drinking habits, honesty, or sexual orientation falls into an uncertain zone between nature and nurture.
Setting up Experiments
Experimental design is a significant challenge in nature-nurture studies, especially with humans. Animal studies, like those involving aggressive and non-aggressive dogs, are more straightforward. These experiments involve breeding and cross-fostering to determine the relative influence of genetics and environment on aggression.
With humans, such controlled experiments are not possible due to ethical constraints. Instead, researchers rely on real-world examples and occurrences to observe nature-nurture dynamics, though these provide only partial answers. Eugenics is an example of unethical practices attempting to shape human characteristics through intentional breeding.
Behavioral Genetics
Behavioral genetics explores how genes and environments interact to influence behavior. Adoption studies offer insights by comparing adopted children to their biological and adoptive parents. These studies can reveal the influence of genetics on traits like height and the environment on traits like language.
Twin studies, comparing monozygotic (MZ) and dizygotic (DZ) twins, provide another avenue for investigation. MZ twins share 100% of their DNA, while DZ twins share 50%. Comparing the similarity of traits between MZ and DZ twins helps to determine the role of genetics. For example, height is very similar in identical twins. The same is not true for fraternal twins. Studying the twin's language, if one identical twin speaks Spanish at home, the co-twin with whom she is raised almost certainly does too (and is the same for fraternal twins).
Quantitative Genetics and Heritability
Quantitative genetics analyzes similarities among individuals based on their biological relatedness (siblings, cousins, twins raised apart). This field produces the heritability coefficient, , a value between 0 and 1 that estimates the influence of genetics on a trait. However, heritability coefficients are often misinterpreted. A heritability coefficient measures how strongly differences among individuals are related to differences among their genes.
Despite the complexity of interpreting heritability coefficients and the quantitative structure, these numbers have a strong influence on the human imagination.
The field has been influenced by genetic discoveries, from Mendel's work to the sequencing of the human genome. However, the implications of this new genetic knowledge for nature-nurture studies remain mysterious.
What We Have Learned About Nature-Nurture
Nature-nurture studies have not provided definitive evidence of the origins of traits. However, the findings suggest that genetics influences nearly every trait to some extent. The more genetically-related people are, the more similar they are (height, weight, intelligence, personality, mental illness, etc.). Even traits like political attitudes, television viewing habits, and divorce rates have a genetic component.
The recognition of genetic influence on behavior is a recent development. Behaviorism and psychoanalysis, dominant in the 20th century, focused on environmental factors and early life experiences, respectively. While these perspectives are not necessarily incompatible with genetics, they often overlooked the role of inherited traits.
A key finding is that adopted children's personalities and mental health are better predicted by their biological parents than their adoptive parents. This challenges purely environmental explanations. However, no behavioral traits are completely inherited, highlighting the importance of both nature and nurture.
Correlation and Causation
In nature–nurture, correlation doesn't imply causation. It's important to note that interpreting a behavior as wholly the result of nurture without further evidence isn't safe when subjects are biologically related. For example, there may be genetic as well as environmental pathways between the parenting practices of mothers and the abilities of their children.
Limitations of Heritability
Nature-nurture studies have not met expectations of organizing traits along a genetic-to-environmental spectrum. Everything has turned out to be at least somewhat heritable, yet nothing has turned out to be absolutely heritable. Also, there hasn’t been much consistency as to which traits are more heritable and which are less heritable once other considerations are taken into account. The heritability coefficient and its underlying quantitative structure don't align with our intuitions. Genes and environment are both crucial to every trait.
The heritability of a trait is not simply a property of that trait, but a property of the trait in a particular context of relevant genes and environmental factors. The classic example of the heritability coefficient defying intuition is the trait of having two arms. Fraternal twins are just as similar for “two-armedness” as identical twins, resulting in a heritability coefficient of zero for the trait of having two arms, but this doesn't mean that arm development is less genetic than we imagine. In fact, the opposite is likely true as the extent that people differ in arm number is likely the result of accidents and, therefore, environmental.
Gene-Environment Interaction
The heritability coefficient divides traits’ determinants into two portions—genes and environment—which are then calculated together for the total variability. This is a little like asking how much of the experience of a symphony comes from the horns and how much from the strings; the ways instruments or genes integrate is more complex than that. Genetic differences can affect behavior under certain environmental circumstances but not others, a phenomenon called gene-environment interaction ().
One example of is from Caspi et al. (2002). It showed that maltreated children with a particular allele of the MAOA gene showed a predisposition to violence. This was not the case for children who had not been maltreated.
Epigenetics, where environmental events modify DNA and transmit those changes to children, further complicates the nature-nurture question.
Susceptibility to Change
Common questions about nature–nurture are: How susceptible is a trait to change? How malleable is it? and Do we “have a choice” about it? For example, phenylketonuria, an inborn error of metabolism, can be treated by avoiding foods containing phenylalanine. Height is also rooted in nature, but the average height has increased significantly in the past 100 years due to changes in diet.
Modern genetics has not provided definitive answers. There are a few rare genes that have been found to have significant effects. Examples: Huntington’s disease can be caused by a single gene. The Apolipoprotein gene can cause early onset dementia.
The genetic impact on behavior is broken up over many genes, each with very small effects. Extreme environmental hardship causes catastrophic effects for many behavioral outcomes, but that is rare.
The Mind-Body Problem and Free Will
The difficulties in solving nature-nurture problems lead to the other great questions about our relationship with the natural world. These question include the mind-body problem and free will. It is tempting to predict that in the future our thinking may need to change when it comes to the effects of genetic differences on all human characteristics. One of the most important things modern genetics has taught us is that almost all human behavior is too complex to be nailed down, even from the most complete genetic information, unless we’re looking at identical twins.
Genetic differences are vital to human moral equality, freedom, and self-determination, not opposed to them. Genetics gets a vote, not a veto, in the determination of human behavior.