Genetics, Evolution & the Nature–Nurture Debate
- Explain how twin and adoption studies untangle genetic from environmental influence
- Define heritability and interpret it correctly
- Apply an evolutionary perspective to explain behavioral tendencies
Nature, nurture, and how to study them
The nature–nurture question asks how much our genes versus our environment shape who we are; today psychologists agree the answer is always both, interacting. To separate the two, researchers use twin studies and adoption studies. Identical (monozygotic) twins share 100% of their genes; fraternal (dizygotic) twins share about 50%, like ordinary siblings. If identical twins are more alike on a trait than fraternal twins, that points to a genetic influence. Adoption studies compare adopted children to their biological versus adoptive families — resemblance to biological relatives signals genes; resemblance to adoptive relatives signals environment.
Heritability: a slippery number
Heritability is the proportion of variation among individuals in a group that we can attribute to genes. The crucial subtlety: heritability describes differences within a population, not any single person. Saying height is 90% heritable does not mean 90% of your height came from genes and 10% from food — it means that within a studied group, most of the variation in height traces to genetic differences. Heritability also changes with the environment: if everyone is well fed, remaining height differences are mostly genetic, so heritability rises.
The evolutionary perspective
Evolutionary psychology explains behavioral tendencies as adaptations that helped ancestors survive and reproduce, favored by natural selection (Darwin). Traits that boosted fitness — surviving and passing on genes — became widespread. This perspective explains universal tendencies like fearing snakes and heights, preferring calorie-dense foods, and forming social bonds. Evolutionary accounts are powerful for broad, species-wide patterns but are criticized for being hard to test and for sometimes offering "just-so" stories after the fact.
A study finds that identical twins raised apart have very similar levels of extraversion, while fraternal twins raised together are much less similar. What does this pattern suggest about the origins of extraversion, and what is one limitation of concluding "genes cause extraversion"?
- 1.Compare the twin types: identical twins share 100% of genes, fraternal twins about 50%.
- 2.Interpret the similarity gap: identical twins are more alike even when raised apart, while fraternal twins raised together are less alike — greater genetic overlap predicts greater similarity.
- 3.Draw the inference: this points to a substantial genetic (heritable) contribution to extraversion.
- 4.State a limitation: heritability describes variation within this population, not a fixed personal percentage; genes and environment interact, and identical twins may also be treated more alike, so genes are not the sole cause.
A psychologist reports that the heritability of intelligence in a well-off, uniform community is 0.75. What does this figure most accurately mean?
The most common heritability error is applying a group statistic to one person. Heritability is about differences among individuals in a population, never the fraction of a single individual’s trait caused by genes. Watch for answer choices that misapply the number to one person.
Researchers want to determine whether shyness is more strongly influenced by genes or by upbringing. Which design most directly separates the two influences?
On nature–nurture FRQs, name the method that isolates the influence: twin studies (identical vs. fraternal) and adoption studies (biological vs. adoptive relatives). Then note that the modern answer is always interaction, not one or the other.
Answer the 2 checkpoints as you read.
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