Nature vs. Nurture in Behavior Genetics
The Nature-Nurture Question: Comprehensive Study Notes
I. Introduction to the Nature-Nurture Question
Core Intuition: People instinctively feel that some behaviors stem from genetics ("nature"), while others result from upbringing or effort ("nurture").
Behavior Genetics: This scientific field seeks to empirically study these differences.
Methods: Examining similarities among family members with varying genetic relatedness or studying DNA differences in people with different behavioral traits.
Challenges: Scientific methods are ingenious but often inconclusive; many difficulties are conceptual, and intuitions become complex upon deeper thought.
Oversimplification: It's an oversimplification to ask how "genetic" a behavior is. Genes and environments always combine to produce behavior; the real science is discovering how they combine for specific behaviors.
Three Fundamental Problems (Philosophy & Science):
Mind-Body Problem: How does the mind relate to the physical body?
Free Will Problem: Do we genuinely have control over our actions?
Nature-Nurture Problem: How do genetics and environment influence our traits and behaviors?
Shared Characteristics of These Problems:
Everyone holds opinions about them, even without scientific or philosophical knowledge.
Our understanding of our relationship with nature feels incomplete (e.g., control over actions vs. mercy to our bodies, consciousness as brain creation vs. awareness beyond physical).
This incompleteness leads to fascination and a degree of obsession.
The "Nature-Nurture Debate": Unique among the three problems for being called a "debate" due to historical controversy and offense in psychology.
Moral Character Connection: Our sense of moral character seems dependent on the nature-nurture explanation.
Examples:
Admiring an athlete's skill but viewing height as a "genetic lottery" gift; no blame for short stature or congenital disability.
Praising a concert violinist's dedication (and parents/teachers) but condemning cheaters or bullies.
Complexity: Most human characteristics are not as clear-cut as height (primarily nature) or instrument mastery (primarily nurture).
Example: A great violinist might have inborn qualities (perfect pitch, nimble fingers) that support hard work, while a tall basketball player might have a diet that promoted genetic tendency for height.
Personal Significance: Traits without obvious causes (e.g., drinking habits, worry, honesty, religiosity, sexual orientation) are often the most personally significant and fall into an uncertain zone between nature and personal control.
II. Empirical Challenges and Research Designs
Difficulty in Human Experimentation: A major problem for answering nature-nurture questions in people is the ethical impossibility of setting up controlled experiments.
Animal Experiments (Feasible):
Example: To study aggression in dogs (Chihuahuas vs. Beagles):
Mate aggressive dogs (Chihuahuas) and nonaggressive dogs (Beagles).
Switch half the puppies from each litter to be raised by the other set of parents.
Observe whether Chihuahua puppies raised by Beagles become nonaggressive, and Beagle puppies raised by Chihuahuas become aggressive.
This determines if nature, nurture, or a combination prevails.
Significance: Much significant nature-nurture research has been done this way (Scott & Fuller, 1998); animal breeders have successfully used these methods for thousands of years.
Limitations for Humans: Techniques used with animals (e.g., random assignment of babies to parents, intentional breeding) cannot be applied to humans due to ethical restrictions (e.g., "eugenics" attempts).
Behavioral Genetics: The science of how genes and environments influence behavior.
Human Research Designs (Observational):
Adoption Studies:
Setup: Children are raised by adoptive parents, distinct from their biological parents.
Insights: Helps differentiate genetic (biological parents) from environmental (adoptive parents) influences.
Examples:
A child of tall biological parents adopted by short parents: would likely still grow tall, suggesting genetic influence on height.
A child of Spanish-speaking biological parents adopted by English-speaking parents: would speak English, suggesting environmental influence on language.
Twin Studies:
Types of Twins:
Monozygotic (MZ) Twins: "Identical" twins; result from a single zygote, share 100\% of their DNA (essentially clones).
Dizygotic (DZ) Twins: "Fraternal" twins; develop from two zygotes, share 50\% of their DNA (like ordinary siblings born at the same time).
Method: Compare the similarity of MZ and DZ pairs for a given trait.
Examples:
Height: Identical twins are almost perfectly similar in height; fraternal twins are more similar than unrelated individuals but not identical. This indicates a strong genetic role in height.
Spoken Language: Both identical and fraternal twins raised together will speak the same language. The higher genetic match of identical twins makes no difference, suggesting a strong environmental role in spoken language.
Quantitative Genetics: A broader class of methods for observing nature-nurture; a scientific discipline that analyzes similarities among individuals based on their biological relatedness.
Applications: Can be used with siblings, half-siblings, cousins, and even extended families.
Rare Cases: Studies of twins separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990) are very rare and play a smaller role than commonly believed.
Heritability Coefficient: A number derived from quantitative genetics, ranging from 0 to 1. It aims to provide a single measure of genetics' influence on a trait.
Interpretation: Measures how strongly differences among individuals are related to differences among their genes.
Caution: Heritability coefficients are simple to compute but deceptively difficult to interpret.
Influence: Numbers providing simple answers to complex questions strongly influence human imagination, leading to extensive debate (e.g., heritability of intelligence, personality, depression).
III. Discoveries and Complexities of Nature-Nurture
Era of Genetic Discovery: Modern genetics is experiencing a period of discovery comparable to Copernicus, Galileo, and Newton's eras for astronomy/physics.
Historical Timeline:
Late 19^{th} century: Francis Galton influenced by Charles Darwin, starts thinking about nature-nurture; genetics per se unknown.
Same time: Mendel's work with peas goes undiscovered for 20 years.
1920s: Quantitative genetics developed.
1950s: DNA discovered by Watson and Crick.
Turn of 21^{st} century: Human genome completely sequenced.
Present: Ability to obtain individual DNA sequences at low cost.
Key Findings on Nature-Nurture:
Genetic Footing for Everything: Contrary to initial hopes for clear genetic/environmental traits, everything has some genetic basis.
The more genetically related people are, the more similar they are across all traits: height, weight, intelligence, personality, mental illness, political attitudes, TV watching, divorce rates (Plomin, Corley, DeFries, & Fulker, 1990; McGue & Lykken, 1992).
Recent Discovery: Genetic influence on behavior is a relatively recent understanding.
Mid-20^{th} Century Dominance: Psychology was dominated by behaviorism (behavior explained solely by environment) and psychoanalysis (roots in early life history).
Historical Oversimplification: It was widely believed children's personalities were shaped entirely by imitation, and schizophrenia was caused by "pathological mothering."
Genes are Essential: Best predictors of an adopted child's personality or mental health are found in biological parents, not adoptive parents. Genes cannot be left out of the equation.
Environment is Essential: No behavioral traits are completely inherited; environment cannot be left out either.
Caution with Environmental Interpretations: When subjects are biologically related, it's never safe to attribute a behavior solely to nurture without further evidence, even if it seems obvious.
Example: Children whose mothers read to them often have better third-grade reading scores. Tempting to conclude reading aloud causes success, but this is inconclusive due to shared genetic and environmental pathways. "Correlation does not imply causation." Requires adoption studies or random assignment for causal claims.
Disappointing Outcomes of Nature-Nurture Studies:
Inability to organize traits on a spectrum from "more-genetic" to "less-genetic."
Everything is somewhat heritable, but nothing is absolutely heritable. No consistency found for which traits are more/less heritable when other factors (e.g., measurement accuracy) are controlled (Turkheimer, 2000).
Conceptual Problems with Heritability Coefficient:
Misconception of "Importance": We want to know how "important" genes and environment are, but both are crucial for every trait (genes need environment, environment needs genes).
Context-Dependence: Heritability looks at differences among people. The cause of a trait depends on the differences within the studied group, not just the trait itself.
Defiance of Intuition: Example: The trait of having two arms.
This is clearly a biological, genetic process.
However, fraternal twins are just as similar for "two-armedness" as identical twins.
Result: Heritability coefficient of 0. This would usually suggest all nurture, no nature.
Explanation: People do not significantly vary in the genes controlling arm development. Differences in arm number are likely due to accidents (environmental factors).
Conclusion: Heritability is a property of the trait in a particular context of relevant genes and environmental factors, not just a property of the trait itself.
Oversimplification of Interaction: The coefficient divides determinants into two portions (genes and environment) and sums them. This is like asking how much of a symphony comes from horns vs. strings; the interaction is more complex.
Gene-Environment Interaction (G x E):
Genetic differences affect behavior under some environmental circumstances but not others.
Example: Caspi et al. (2002) showed that among maltreated children, those with a specific allele of the MAOA gene had a predisposition to violence; those with other alleles did not. In non-maltreated children, the gene had no effect.
Epigenetics: Recent studies show DNA itself can be modified by environmental events, and these genetic changes can be transmitted to offspring.
Multiple Answers: The answer to the nature-nurture question is not straightforward. Different questions about genes, environments, and traits may have different answers, and the answer to one tells little about the others.
Questions include: trait susceptibility to change, malleability, conscious choice, influence of parents vs. culture, sensitivity to genetic differences, range of relevant genes, and whether the trait is best described genetically or behaviorally.
IV. Implications and Future Directions
Unanswered Questions: Even modern genetics has not provided definitive answers.
Few "Single Genes of Great Effect": While rare genes like those for Huntington's disease or some Alzheimer's cases (Apolipoprotein gene) have significant (often negative) effects, most behavioral traits are influenced by many genes, each with very small effects.
Diffuse Genetic and Environmental Impact: Genetic impact is distributed across too many genes to be meaningfully cataloged. Similarly, within the normal range of environmental events, the specific factors causing differences are hard to grasp.
Connection to Mind-Body and Free Will:
Just as consciousness isn't from one brain area, and choice isn't an orderly activity, nature-nurture becomes more complex upon closer inspection.
Societal Impact (Exaggerated Hopes/Fears):
Predictions: Some predict profound changes in cultural, ethical, legal, and personal thinking (e.g., considering genetic background in criminal proceedings, difficult reproductive decisions for parents with genetic sequences).
Reality: Most human behavior is too complex to be fully determined even by complete genetic information, except for identical twins.
Genetic Diversity and Equality: Modern genetics emphasizes that genetic differences among people are vital to human moral equality, freedom, and self-determination, not opposed to them.
"Genetics Gets a Vote, Not a Veto" (Mordecai Kaplan): Genetics contributes to behavior, but it does not solely dictate it.
Conclusion: Indulge fascination with nature-nurture while resisting oversimplification.