Genetic Similarity ERQ

Genetic similarity

The degree to which individuals share the same genetic material, such as monozygotic twins (100%) or dizygotic twins (50%).

Twin studies

By comparing concordance rates in MZ and DZ twins, researchers estimate how much behaviour is influenced by genes versus environment.

Heritability

A statistical estimate of the proportion of behavioural variation in a population that is due to genetic differences, often calculated using concordance rates

Aim of McGue et al. (2000)

To investigate genetic and environmental influences on adolescent tobacco and marijuana use.

McGue et al participants

626 pairs of male and female twins born in the same year. Males: 188 MZ and 101 DZ. Females: 223 MZ and 114 DZ.

Method of McGue et al. (2000)

Participants were interviewed using structured interviews about their lifetime use of legal (tobacco) and illegal (marijuana) substances. Participants also completed questionnaires assessing frequency of drug use and aspects of their home environment, including parental drug use and family attitudes towards substances. Concordance rates between MZ and DZ twins were analysed to estimate heritability and the influence of shared environmental factors.

Key findings of McGue et al.

Marijuana use showed low heritability (10–25%) with no sex differences. Tobacco use showed higher heritability (40–60%). Shared environment was strongly linked to drug use.

Conclusion of McGue et al.

Environmental factors, particularly family exposure to drugs, appeared more influential than genetics in determining adolescent drug use.

Strengths of McGue et al.

Large sample, clear twin design, practical insights for drug prevention, balanced view of genes and environment. Both illegal and legal substances increases breadth.

Limitations of McGue et al.

Self-report bias, correlational design, focus limited to two drugs, cultural/time-specific context.

Aim of Kendler et al. (2006)

To investigate the heritability of Major Depressive Disorder (MDD) and explore sex differences in heritability.

Participants in Kendler et al. (2006)

15,493 complete twin pairs (MZ and DZ) from the Swedish national twin registry, including both male and female twins.

Method used in Kendler et al. (2006)

Large-scale twin study using telephone interviews conducted over 4 years; diagnosis based on DSM-IV criteria and antidepressant prescriptions.

Procedure of Kendler et al. (2006)

Twins were interviewed about depressive symptoms and medication history. Concordance rates were calculated and compared for MZ and DZ twins. Time spent living together was also analyzed.

Main results of Kendler et al. (2006)

Average heritability of MDD was 38%. MZ twins had higher concordance than DZ twins. Female MZ: 44%, Female DZ: 16%, Male MZ: 31%, Male DZ: 11%. Time spent living together had no influence.

Conclusion of Kendler et al.

MDD is influenced by genetics, with higher heritability in women. Environmental factors also contribute, and sex-specific genetic risks may exist.

Strengths of Kendler et al.

Very large, representative sample; reliable diagnostic criteria; shows genetic influence with sex-based nuance; ruled out shared environment effects.

Limitations of Kendler et al.

Doesn't identify specific genes; cannot establish cause-effect; only measures concordance without biological mechanisms.

Support for genetic similarity in behaviour

Both studies show that greater genetic similarity (MZ twins) leads to higher concordance in behaviours like drug use and depression, suggesting genetic influence. High ecological validity as it demonstrates naturally occuring genetic similarity.

Extent of genetic similarity's effect on behaviour

It plays a significant role, but not in isolation. Environmental context and individual experiences shape how genetic predispositions manifest. Experiments are only correlational and not causational, not ethically possible to manipulate experimentally.

Conclusion of the ERQ

Genetic similarity influences behaviours like addiction and depression, but behaviour is shaped by gene-environment interaction and varies by context and individual differences.