# Nature vs. Nurture

Psychology I · Individual Differences · https://tryals.app/learn/psychology-i/nature-vs-nurture

## Untangling Genes and Environment

The **nature versus nurture** debate asks how much individual variation in traits like intelligence comes from genetic inheritance versus environmental experience. Behavioral genetics measures this using **twin studies**.

| Twin Type | Genetic Share | Meaning |
| --- | --- | --- |
| **Monozygotic (identical)** | ~100% | Compare to estimate genes |
| **Dizygotic (fraternal)** | ~50% | Share like siblings |

If identical twins resemble each other much more than fraternal twins, that gap suggests a genetic contribution. **Heritability** is the proportion of variance in a trait within a population attributable to genetic differences.

$$\text{Heritability} \approx 2 \times (r_{identical} - r_{fraternal})$$

For adult IQ, heritability is estimated at 50-80%, often rising with age as people select environments fitting their predispositions.

## Heritability Pitfalls

Heritability is widely misunderstood. Two clarifications matter enormously when interpreting these statistics.

First, heritability describes variance **within a specific population**, not an individual. A heritability of 70% does NOT mean 70% of your personal intelligence comes from genes.

Second, heritability within a population says **nothing about differences between groups**, since those groups may differ systematically in environmental factors.

> **Common pitfall:** Interpreting heritability as a percentage breakdown of a single person's traits. It is strictly a population-level statistic about the source of variance across many people.

## Practice questions

5 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Select every statement that is TRUE about behavioral genetics.

A. Heritability is a population-level statistic about variance.
B. Fraternal twins share about 50% of their genes, like ordinary siblings.
C. The genetic contribution to IQ appears to strengthen from childhood into adulthood.
D. Identical twins share about 100% of their genes.
E. A figure of 70% tells you how much of YOUR OWN intelligence came from your genes.

**Answer:** A. Heritability is a population-level statistic about variance.; B. Fraternal twins share about 50% of their genes, like ordinary siblings.; C. The genetic contribution to IQ appears to strengthen from childhood into adulthood.; D. Identical twins share about 100% of their genes.

**Why:** Identical twins' ~100% and fraternal twins' ~50% gene sharing, the rising heritability with age, and heritability as a population statistic are all true. Heritability does NOT mean 70% of an individual's trait is genetic - the one false option.

Page: https://tryals.app/practice/psychology-i/nature-vs-nurture/select-every-statement-that-is-true-about-behavioral-genetics

### 2. A child with a genetic tendency toward reading ability is read to more, chooses more books, and is put in a better class. Why does this make heritability and environment hard to separate?

A. Because heritability estimates assume environments are randomly assigned, which twin studies violate
B. Because the environmental effect is too small to measure alongside the genetic one
C. Because reading ability is unusual in being both genetic and learned
D. Because the genotype is helping to select the environment

**Answer:** D. Because the genotype is helping to select the environment

**Why:** The genotype is not sitting in a randomly assigned environment, it is helping to choose one, evoking responses and selecting niches. So a share of what the estimate books as environment was set in motion by genes, and the clean split between nature and nurture never quite exists to be measured.

Page: https://tryals.app/practice/psychology-i/nature-vs-nurture/a-child-with-a-genetic-tendency-toward-reading-ability-is-read-to

### 3. Sort each design feature by the twin-study comparison it supports.

**Answer:**

- Compares identical vs fraternal twins: A larger identical-fraternal similarity gap suggests a genetic contribution, Uses the 50% vs 100% gene-sharing difference
- Studies identical twins raised apart: Any similarity cannot be explained by a shared upbringing, Holds genes constant while environments differ

**Why:** Comparing identical (100%) and fraternal (50%) twins uses their gene-sharing gap to estimate genetic contribution; studying identical twins raised apart holds genes constant while varying environment, so any similarity cannot be due to shared upbringing. The two designs provide complementary evidence.

Page: https://tryals.app/practice/psychology-i/nature-vs-nurture/sort-each-design-feature-by-the-twin-study-comparison-it-supports

### 4. IQ heritability within a population is often estimated around 50-70%. Why does that figure say almost nothing about why two populations might differ in average IQ?

A. Because heritability estimates are too imprecise to support any conclusion about anything.
B. Because a heritability of 70% leaves 30% environmental, which is too small to explain group differences.
C. Because heritability applies only to physical traits, and its use for IQ is invalid.
D. Because heritability describes variation within a group, not between.

**Answer:** D. Because heritability describes variation within a group, not between.

**Why:** Lewontin's seeds make it unmistakable: split a varied batch between rich and depleted soil, and within each pot height differences are almost entirely genetic while the difference between pots is entirely soil. Heritability is a variance ratio computed inside one population in one range of environments, and the Flynn effect is the clinching illustration.

Page: https://tryals.app/practice/psychology-i/nature-vs-nurture/iq-heritability-within-a-population-is-often-estimated-around-50-70

### 5. A heritability estimate of 50% within one population tells you that about half of any average difference between two populations is genetic.

**Answer:** False

**Why:** False. Heritability describes variance *within* the population measured and carries no implication about between-group differences. Lewontin's seed-lot analogy makes it concrete: identical seeds in rich and poor soil give high within-plot heritability and a between-plot difference that is entirely environmental.

Page: https://tryals.app/practice/psychology-i/nature-vs-nurture/a-heritability-estimate-of-50-within-one-population-tells-you-that