Genetic similarity
Genetic similarity refers to the proportion of shared genetic material between individuals.
Heritability
The proportion of variation in a trait attributed to genetic differences within a population, often expressed as a percentage.
Falconer Model
A statistical method to calculate heritability by comparing behavioral similarities in MZ and DZ twins.
Gene Expression
The process by which information from a gene is used to produce functional products, influenced by both genetic and environmental factors.
Methods of Study
Twin Studies
- Twin studies are fundamental to understanding the genetic basis of behavior. There are two types of twins:
- Monozygotic (MZ) Twins: Identical twins who share 100% of their DNA.
- Dizygotic (DZ) Twins: Fraternal twins who share approximately 50% of their DNA, similar to regular siblings.
- By comparing MZ and DZ twins, researchers infer the extent of genetic influence on specific traits.
- Greater similarity in MZ twins compared to DZ twins suggests a stronger genetic component.
- Results are often given as a concordance rate, which ranges between 0 (no similarity) and 1 (perfect similarity)
Adoption Studies
- Adoption studies separate genetic and environmental influences by studying individuals raised apart from biological relatives.
- Comparisons between adopted children and their biological and adoptive families help identify contributions of genes and upbringing.
Family Studies
- Examine behavior across several generations, exploring how traits are passed down.
- Include a broader genetic spectrum, such as siblings, cousins, parents, and grandparents.
Molecular Genetics
- This approach identifies specific genes associated with behaviors through genetic mapping and epigenetic studies.
- Explains how certain genes are expressed or suppressed in response to environmental factors.
Key Studies
Twin Study on Antisocial Behavior
Case studyBaker et al., 2007
Aim: To investigate the heritability of antisocial behavior in children.
Method: Twin study using correlational analysis.
Participants: Over 1,000 pairs of MZ and DZ twins and triplets from Southern California, representing diverse socioeconomic and ethnic backgrounds.
Procedure: Antisocial behavior (including aggression, psychopathy, and conduct issues) was assessed using self-reports, caregiver questionnaires, and teacher reports.
Results:
Heritability estimates:
- Caregiver reports: 67%.
- Self-reports: 42%.
- Teacher reports: 55%.
- Average heritability: ~50%.
Conclusion: Antisocial behavior in children is influenced equally by genetic and environmental factors. However, extreme behaviors, such as severe aggression or psychopathy, showed higher genetic influence, highlighting the importance of understanding both hereditary and environmental contexts.
Adoption Study on Antisocial Behavior
Case studyGrove et al., 1990
Aim: To determine the heritability of antisocial behavior using separated identical twins.
Method: Adoption study.
Participants: 32 pairs of MZ twins raised in different families.
Procedure: Researchers assessed antisocial tendencies in childhood and adulthood using behavioral tests and observations.
Results:
- Childhood heritability: 41%.
- Adulthood heritability: 28%.
Conclusion: Antisocial behavior is significantly influenced by genetics during childhood but decreases in adulthood as environmental factors play a larger role. This highlights the dynamic nature of genetic and environmental interactions over time.
Meta-Analysis of Twin and Adoption Studies
Case studyMason, 1994
Aim: To analyze the heritability of antisocial behavior across multiple studies.
Method: Meta-analysis of 12 twin and 3 adoption studies conducted from 1975 to 1994.
Findings:
- Heritability of antisocial behavior: ~50%.
- Extreme antisocial behaviors showed stronger genetic influences.
Conclusion: While antisocial behavior generally results from a mix of genetic and environmental factors, extreme cases are more likely to be genetically driven.
Twin Study on Intelligence
Case studyBouchard and McGue, 1981
Aim: To estimate the heritability of intelligence using twin data.
Method: Meta-analysis of 111 twin studies.
Participants: Included MZ and DZ twins (both reared together and apart), siblings, and parents.
Procedure: IQ correlations between different pairs of relatives were calculated, and the Falconer Model was applied to estimate heritability.
Results:
- MZ twins reared together: IQ correlation = 0.85.
- MZ twins reared apart: IQ correlation = 0.67.
- DZ twins reared together: IQ correlation = 0.58.
- Heritability of intelligence: 54%.
Conclusion: Intelligence is significantly influenced by genetic factors but also shaped by environmental inputs.
The 5-HTT Gene and Stress
Case studyCaspi et al., 2003
Aim: To explore the role of the 5-HTT gene in modulating stress-induced depression.
Method: Longitudinal study with genetic mapping.
Participants: 1,037 individuals from New Zealand.
Procedure: Participants were divided into three genetic groups (two short alleles, one short and one long allele, two long alleles). Life events and depression symptoms were tracked from ages 3 to 26.
Results:
- Individuals with one or two short alleles were more vulnerable to stress-induced depression.
- Those with long alleles were less likely to develop depression despite experiencing similar stressors.
Conclusion: Genetic predispositions, such as the presence of the short 5-HTT allele, influence how individuals respond to environmental stressors, highlighting a gene-environment interaction.
