There is a significant body of work examining the genetic contributions to many different human traits. Generally, researchers find that identical twins are more similar to one another than are fraternal twins--for example, they are more similar on gun ownership, voting preferences, homosexuality, job satisfaction, coffee consumption, rule enforcement and insomnia. Such findings have been interpreted to mean that:

Question

BlackTom AI · Accepted Answer

Higher concordance among identical (monozygotic, MZ) twins compared with fraternal (dizygotic, DZ) twins signals a genetic influence on the traits.

1. Relevant knowledge points and concepts
- MZ twins share ~100% of their genes; DZ twins share ~50% on average.
- Heritability estimates how much of population variation in a trait is due to genetic differences.
- Twin-study logic: if MZ twins are more similar than DZ twins, genetic factors contribute to that similarity (assuming comparable environments).
- Falconer’s formula (a common back-of-the-envelope estimator): h^2 ≈ 2(r_MZ − r_DZ).

2. Complete reasoning process step by step
- Empirical observation: across many traits (e.g., voting, insomnia), MZ twins show higher concordance than DZ twins.
- Because the main systematic difference between MZ and DZ pairs is genetic similarity (100% vs ~50%), the extra similarity of MZ pairs is attributed to genes.
- This pattern, repeated for many traits, implies genetics play a role in those traits.

3. Calculations and intermediate steps (example)
- Suppose correlation for MZ twins on trait X is r_MZ = 0.6; for DZ twins r_DZ = 0.3.
- Falconer’s estimate: h^2 = 2(r_MZ − r_DZ) = 2(0.6 − 0.3) = 2(0.3) = 0.6.
- Interpretation: ~60% of variation in this trait (in that sample) is attributable to genetic differences.

4. Why the correct answer is correct
- The observed consistent pattern (MZ > DZ similarity) provides direct evidence of genetic contribution to variance in many traits; thus concluding a genetic component to virtually all traits is justified.

5. Why other options are incorrect
- “We can't know the genetic contribution”: incorrect — twin studies provide estimates of genetic contribution.
- “Identical twins are more similar because they have more shared experiences”: this is a challenge (equal-environments assumption) but does not account for the consistent widespread MZ > DZ pattern; studies control for this and still find genetic effects.
- “The environment has little influence”: wrong — heritability does not imply no environmental influence; many traits show both genetic and environmental contributions.

6. Conclusion
These points confirm the correct answer: there is a genetic component to virtually all of our traits.

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