The role of genetics in influencing behaviour

Genetics plays a significant part in shaping human behaviour through the interaction of inherited characteristics and environmental factors. Each individual has a unique genetic code that sets the potential for certain abilities, but the realisation of these potentials depends on external influences.

Key concepts in genetics and behaviour

• Genotype - The genetic make-up of an individual, which forms the blueprint for potential traits and abilities.
• Phenotype - The observable characteristics of an individual resulting from the interaction between their genotype and environmental experiences.
• Genetic expression - The process by which genes influence development and behaviour, activated only when triggered by environmental factors. This can lead to both physiological changes (like hormone levels) and psychological effects (like emotional responses).
• Cumulative genetic influence - Rather than a single gene causing a specific behaviour, multiple genes often contribute small effects that combine to impact behaviour overall.

Genetic influence on aggression

Aggression is one behaviour believed to have a genetic basis.

Aggression is mediated through various biological mechanisms:

• Hormone regulation - Certain genes affect how hormones like testosterone, serotonin, dopamine, and adrenaline are metabolised in the body, influencing aggressive tendencies.
• Brain function - Genes impact the frontal lobe of the brain by altering how receptors process testosterone in neurons, potentially reducing impulse control and increasing aggression when exposed to provoking environmental cues.
• Monoamine oxidase A (MAOA) gene - This gene is linked to aggression by regulating neurotransmitter levels in the brain. The MAOA enzyme, controlled by this gene, breaks down neurotransmitters. Individuals with a low-activity variant of the MAOA gene often show higher aggression due to elevated neurotransmitter levels.

Study methods for assessing genetic contributions to behaviour

Researchers use several scientific approaches to investigate how much genetics contributes to specific behaviours. These methods help distinguish between inherited traits and environmental influences.

Methods used to study genetic influences

• Twin studies - Monozygotic (MZ) twins share 100% of their genetic material, while dizygotic (DZ) twins share only 50%. If a particular behaviour or ability is more common among MZ twins than DZ twins, it indicates a genetic component to that trait.
• Adoption studies - Adopted children share genes with their biological parents but grow up in the environment of their adoptive parents. If a behaviour resembles that of biological parents more than adoptive parents, it suggests a genetic influence.
• Gene mapping - This method identifies specific genes shared by individuals with high levels of a particular ability or behaviour, compared to those without it, indicating a genetic association with that trait.

Key research on the MAOA gene and aggression

Numerous studies have explored the link between the MAOA gene and aggressive behaviour, providing evidence for genetic influences while highlighting the role of environmental triggers.

Brunner et al. (1993) study on MAOA gene mutation

• Method - A case study focused on a Dutch family with a history of aggressive behaviour. Urine samples were collected to measure monoamine levels.
• Results - Five male family members had high monoamine levels due to a shortened variant of the MAOA gene, unique to males, which resulted in low MAOA enzyme activity and poor neurotransmitter breakdown. These males displayed extreme aggression, including acts like arson and violence, especially when provoked or frustrated.
• Conclusions - A connection exists between genetic mutations in the MAOA gene, altered brain chemistry, and aggressive behaviour.
• Evaluation - As a case study of a single family, findings may not apply broadly to the general population. Additionally, it does not explain why some males with the same gene variant do not show increased aggression.

Moffitt (1992) longitudinal study on MAOA and abuse

• Method - A long-term study followed 442 males in New Zealand from birth, examining the interaction between the MAOA gene variant and childhood experiences.
• Results - Males with the shortened MAOA gene who experienced childhood abuse were nine times more likely to exhibit anti-social behaviour, including aggression, compared to those without abuse.
• Conclusions - The MAOA gene may predispose individuals to aggression, but this behaviour often requires an environmental trigger like abuse to be expressed.
• Evaluation - The longitudinal design provides strong evidence of cause and effect over time, though it focuses only on males and may miss other contributing factors.

Cases et al. (1995) study on genetically engineered mice

• Method - Mice were genetically modified to have low MAOA levels, and their behaviour was observed.
• Results - These mice showed abnormal serotonin levels and increased aggression as adults.
• Conclusions - Supports human research linking low MAOA activity to aggression, suggesting a biological basis across species.
• Evaluation - While animal studies offer controlled insights, direct application to human behaviour is limited due to species differences.

Crisp et al. (1985) study on anorexia nervosa

• Method - Investigated concordance rates for anorexia nervosa among MZ and DZ twins.
• Results - Found a 55% concordance rate among MZ twins compared to only 7% among DZ twins.
• Conclusions - Suggests a significant genetic component to anorexia nervosa, highlighting that genetics can influence a range of behaviours beyond aggression.
• Evaluation - High concordance among MZ twins supports genetic influence, but the lower rate in DZ twins indicates environmental factors also play a role.

Evaluations of genetic explanations for behaviour

While genetic research provides valuable insights into behaviour, it is not without limitations. Both strengths and weaknesses must be considered when interpreting these explanations.

Strengths of genetic explanations

• Diathesis-stress model - Explains why not all individuals with certain gene variants, like the shortened MAOA gene (present in about one-third of men), display related behaviours. The model suggests a genetic predisposition (diathesis) combined with environmental triggers (stress) is necessary for behaviours like aggression to manifest.
• Advancements in DNA testing - Modern technology allows psychologists to identify specific genes linked to behaviours, enhancing the precision of research and potential interventions.

Limitations of genetic explanations

• Incomplete genetic determinism - Behaviours like aggression cannot be fully explained by genetics alone. If they were, MZ twins would show 100% concordance rates for such traits, which is rarely the case, indicating environmental influences are also crucial.
• Gender-specific limitations - The shortened MAOA gene variant is male-specific, so it cannot account for aggression in females or in males without this variant.
• Alternative theories - Non-biological explanations, such as de-individuation (where individuals lose self-control in group settings), have substantial research support, suggesting psychological and social factors also contribute significantly to behaviours like aggression.

Issues and debates in genetic research

• Nature vs. nurture - If behaviour were entirely due to genes, it would support genetic determinism (nature). However, evidence suggests interactionism, where both nature (biological factors like genes) and nurture (environmental influences) combine to shape behaviour.
• Complexity of behaviour - Most behaviours result from a dynamic interplay of multiple genetic and environmental factors, rather than a single cause.

Practical applications and ethical considerations of genetic research

Genetic research into behaviour has real-world implications, offering potential benefits but also raising significant ethical concerns.

Applications of genetic research

• Identification through DNA testing - DNA testing can pinpoint individuals with genetic predispositions to certain behaviours, such as aggression, potentially allowing for early interventions.
• Genetic modification - There is the possibility of altering genes to reduce the likelihood of undesirable behaviours, which could have profound societal benefits if applied responsibly.

Ethical concerns

• Risk of misuse - Genetic screening could lead to controversial decisions, such as aborting foetuses based on perceived genetic risks for undesirable traits, raising serious moral questions.
• Stigmatisation - Labelling individuals based on genetic profiles might result in discrimination or unfair treatment, even if the behaviour never manifests.
• Consent and privacy - Using genetic data requires careful handling to protect individual rights and prevent misuse of sensitive information.