Genetics and Behaviour

Introduction

The question of how much behaviour is determined by genes versus environment is one of the oldest And most contentious in psychology. Behaviour genetics is the scientific study of the relative Contributions of genetic and environmental factors to individual differences in behaviour. Modern Behaviour genetics has moved beyond simple nature-versus-nurture debates to investigate how genes And environment interact to shape behaviour throughout the lifespan.

Research Methods in Behaviour Genetics

Twin Studies

Twin studies compare the similarity of monozygotic (MZ, identical) twins, who share approximately 100% of their genes, with dizygotic (DZ, fraternal) twins, who share approximately 50% of their Segregating genes on average. The logic is straightforward: if MZ twins are more similar than DZ Twins on a given trait, this greater similarity can be attributed to genetic factors, because both Types of twins share the same family environment (assuming equal environments assumption holds).

Heritability estimates are calculated by doubling the difference between the MZ and DZ Correlations:

$h^2 = 2(r_{MZ} - r_{DZ})$

Where $h^2$ is the heritability coefficient, $r_{MZ}$ is the correlation for MZ twins, and $r_{DZ}$ Is the correlation for DZ twins.

Strengths of twin studies:

• Provide a natural experiment for estimating genetic and environmental contributions.
• Allow researchers to estimate both heritability and shared environmental effects.
• Can be combined with longitudinal designs to study how genetic and environmental influences change over the lifespan.

Limitations of twin studies:

• The equal environments assumption: MZ twins may experience more similar environments than DZ twins (e.g., they are more often dressed alike, treated more similarly by parents, and spend more time together). If so, greater MZ similarity could reflect environmental rather than genetic factors.
• MZ twins are not genetically identical. Post-zygotic mutations, epigenetic differences, and somatic mosaicism mean that even “identical” twins have some genetic differences.
• Twins may not be representative of the general population. Twin pregnancies have higher rates of complications, and twins may have different developmental trajectories than singletons.

Adoption Studies

Adoption studies compare adopted children with their biological parents (with whom they share genes But not environment) and with their adoptive parents (with whom they share environment but not Genes). This design allows researchers to separate genetic and environmental influences more cleanly Than twin studies.

Strengths of adoption studies:

• Directly separate genetic and environmental influences by comparing biological and adoptive relatives.
• Avoid the equal environments assumption that plagues twin studies.
• Can reveal gene-environment interactions by examining how genetic predispositions are expressed in different adoptive environments.

Limitations of adoption studies:

• Selective placement: Adoption agencies often try to place children in families similar to their biological families (in terms of socioeconomic status, education, etc.), which can inflate estimates of shared environmental influence.
• Prenatal environment: Adopted children share the prenatal environment (and often the first months of postnatal life) with their biological mothers, which can confound genetic and prenatal environmental influences.
• Adoption itself is a unique environmental experience that may not generalise to the broader population.

The Genetic Basis of Behaviour

Heritability

Heritability ($h^2$) is a statistical measure of the proportion of phenotypic variance in a Population that can be attributed to genetic variance. It ranges from 0 to 1, where 0 indicates no Genetic contribution and 1 indicates that all variance is genetic.

Critical points about heritability that are frequently misunderstood:

1. Heritability is a population statistic, not an individual one. It tells us nothing about the genetic contribution to any particular individual”s behaviour. Saying that intelligence is 50% heritable does not mean that 50% of any individual’s intelligence is due to genes.
2. Heritability can vary across populations and environments. The same trait can have different heritability estimates in different populations or at different times. For example, in a population where everyone receives adequate nutrition, most variance in height is genetic (high heritability). In a population where nutrition varies widely, environmental factors account for more variance (lower heritability).
3. High heritability does not mean immutability. A trait with high heritability can still be strongly influenced by environmental interventions. Phenylketonuria (PKU) is a genetic disorder caused by a single gene, but its effects (intellectual disability) can be entirely prevented by a dietary intervention.
4. Low heritability does not mean the environment is unchangeable. A trait with low heritability could still be difficult to change through environmental intervention if the relevant environmental factors are themselves stable or difficult to modify.

Estimates of Heritability for Psychological Traits

Gene-Environment Interaction

Gene-environment interaction (GxE) occurs when the effect of genetic predispositions on behaviour Depends on the environment, or conversely, when the effect of environmental factors depends on Genetic makeup. GxE means that genetic and environmental factors do not add together; they Combine in non-additive ways.

Caspi et al. (2003): 5-HTTLPR and Stress

One of the most influential studies of gene-environment interaction was conducted by Caspi, Sugden, Moffitt, and colleagues. They investigated whether a functional polymorphism in the promoter region Of the serotonin transporter gene (5-HTTLPR) moderates the effect of stressful life events on Depression.

Methodology:

• Participants were a birth cohort of 847 individuals followed from age 3 to age 26.
• Genetic data were collected (participants were genotyped for the 5-HTTLPR polymorphism, which has two common alleles: short (s) and long (l)).
• Stressful life events (e.g., employment, financial, housing, health, and relationship stressors) occurring between ages 21 and 26 were assessed.
• Depression was assessed using a structured clinical interview.

Findings:

• Participants with one or two short alleles (s/l or s/s genotype) who experienced multiple stressful life events had a significantly higher probability of developing depression than participants with two long alleles (l/l genotype) who experienced the same number of stressful life events.
• Among participants who experienced no stressful life events, there was no difference in depression rates between genotypes.
• The short allele was not a “depression gene” — it was a “susceptibility allele” that increased vulnerability to depression only in the context of adverse life events.

Evaluation:

• The study was a landmark in demonstrating that genetic effects on complex behaviours cannot be understood in isolation from environmental context.
• However, subsequent meta-analyses have produced mixed results. A large meta-analysis by Risch et al. (2009) failed to replicate the GxE interaction, finding no significant difference in depression rates between genotypes at any level of stress.
• The discrepancy may be due to methodological differences in how stress and depression were measured, differences in sample characteristics, or the complexity of the genetic architecture (5-HTTLPR is just one of many genes involved in serotonin regulation).
• The debate highlights the difficulty of replicating GxE findings in psychiatric genetics and the need for very large sample sizes.

Epigenetics

Epigenetics is the study of heritable changes in gene expression that do not involve changes to the DNA sequence itself. Epigenetic mechanisms provide a molecular explanation for how environmental Experiences can produce lasting changes in gene expression and behaviour.

Mechanisms of Epigenetic Regulation

1. DNA methylation: The addition of a methyl group ($\mathrm{CH}_3$) to cytosine bases, at CpG sites (where cytosine is followed by guanine). Methylation generally silences gene expression by preventing transcription factors from binding to the DNA or by recruiting proteins that compact the chromatin structure, making the DNA inaccessible.
2. Histone modification: Histones are proteins around which DNA is wound. Chemical modifications to histone tails (acetylation, methylation, phosphorylation) alter the chromatin structure, making DNA more or less accessible to transcription machinery. Acetylation generally opens the chromatin (activating gene expression), while methylation can either activate or repress expression depending on the specific modification.
3. Non-coding RNA: Small RNA molecules that regulate gene expression post-transcriptionally by binding to messenger RNA (mRNA) and preventing translation or promoting degradation.

Meaney’s Rat Studies

Michael Meaney and colleagues conducted a seminal series of studies demonstrating that maternal care In rats produces epigenetic changes in offspring that persist into adulthood and affect stress Responses.

Key findings:

• Pups of “high-licking and grooming” (LG) mothers showed reduced stress responses (lower corticosterone levels, faster return to baseline) compared to pups of “low-LG” mothers.
• Cross-fostering experiments demonstrated that the effect was due to maternal behaviour, not genetics: pups born to low-LG mothers but raised by high-LG mothers showed the low-stress phenotype, and vice versa.
• At the molecular level, high maternal care produced increased glucocorticoid receptor (GR) expression in the hippocampus. GRs are important for negative feedback regulation of the HPA axis (the body’s stress response system). More GRs mean more effective negative feedback, meaning the stress response is turned off more quickly.
• The epigenetic mechanism involved decreased methylation of the promoter region of the NR3C1 gene (which encodes the glucocorticoid receptor) in the hippocampus. Reduced methylation allowed greater GR expression.
• These epigenetic changes persisted into adulthood and were transmitted to the next generation: female offspring of high-LG mothers became high-LG mothers themselves, perpetuating the cycle.

Evaluation:

• The cross-fostering design elegantly disentangles genetic and environmental influences, providing strong evidence that maternal behaviour causes epigenetic changes in offspring.
• The findings have important implications for understanding the biological mechanisms by which early life experiences shape long-term mental health outcomes.
• Generalisability from rats to humans must be treated with caution. However, analogous findings have been reported in humans: McGowan et al. (2009) found increased methylation of the NR3C1 gene promoter in the hippocampi of suicide victims with a history of childhood abuse, compared to suicide victims without abuse histories and to controls.
• Epigenetic changes are potentially reversible, offering hope for therapeutic interventions. Weaver et al. (2004) demonstrated that administering a histone deacetylase inhibitor to adult rats could reverse the epigenetic changes and behavioural effects of low maternal care.

Bouchard et al. (1990): Minnesota Twin Study

Thomas Bouchard and colleagues at the University of Minnesota conducted the most comprehensive study Of reared-apart twins, recruiting 100 pairs of MZ twins who had been separated early in life and Raised in different environments. The study included extensive psychological, physiological, and Medical assessments.

Key findings:

• MZ twins reared apart showed remarkable similarity on a wide range of measures, including intelligence (correlation approximately 0.75), personality traits (correlations of approximately 0.50 for most Big Five traits), and physiological measures (heart rate, blood pressure, EEG patterns).
• The correlations for MZ twins reared apart were only slightly lower than those for MZ twins reared together, suggesting that shared family environment has relatively little influence on most psychological traits.
• Some striking individual cases were reported: the “Jim twins” (Jim Lewis and Jim Springer), separated at 4 weeks of age and reunited at age 39, showed extraordinary similarities including identical names for their sons (James Alan and James Allan), identical occupations (both were security guards), identical hobbies (both were keen woodworkers), and both had married women named Linda and then Betty.

Evaluation:

• The large sample size and comprehensive assessment battery make this one of the most important studies in behaviour genetics.
• The study supports the conclusion that genetic factors account for a substantial proportion of variance in intelligence, personality, and other psychological traits.
• Limitations include selective placement (some twins were placed in similar environments), the representativeness of twins who volunteer for such studies, and the retrospective nature of some data collection.
• The striking individual cases (such as the Jim twins) are anecdotal and should not be given disproportionate weight relative to the quantitative data.

For an overview of biological topics, see Biological Level of Analysis.

Common Pitfalls

1. Confusing genotype (genetic makeup) with phenotype (observable characteristics).

2. Forgetting that a recessive allele can be present in a carrier without being expressed in the phenotype.

3. Writing vague answers without specific biological terminology — use precise terms (e.g., ‘phospholipid bilayer’ not ‘membrane’).

4. Forgetting to include control variables in experimental design, leading to invalid conclusions.

5. Failing to link structure to function when describing biological molecules, cells, or organs.

6. Confusing DNA replication (S phase) with transcription or translation — be clear about which process is being described.

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.