Neural & Hormonal Mechanisms in Aggression (AQA A-Level Psychology): Revision Notes
Neural & Hormonal Mechanisms in Aggression
Introduction to aggression
Aggression represents one of the most pressing social issues in contemporary society. Psychologists distinguish between two main types of aggressive behaviour. Proactive aggression refers to calculated, 'cold-blooded' behaviour aimed at achieving specific goals through planning. In contrast, reactive aggression describes 'hot-blooded' responses characterised by anger, impulsivity, and accompanying physiological arousal.
Reactive aggression has attracted greater research attention due to its association with more social problems and its biological underpinnings. This focus on reactive aggression forms the basis for much of the biological research discussed in this topic.
Neural mechanisms in aggression
The limbic system
The limbic system comprises subcortical brain structures including the hypothalamus, amygdala, parts of the hippocampus, thalamus, cingulate gyrus, septal area, and fornix. This network plays a central role in regulating emotional behaviour, particularly aggression.
The amygdala represents the most important structure within this system for understanding aggressive behaviour. It functions as a threat detection centre, evaluating environmental stimuli and determining appropriate responses to perceived challenges. Research demonstrates that heightened amygdala reactivity serves as a reliable predictor of aggressive behaviour in both humans and animals.
Key Study: Gospic et al. (2011) - Ultimatum Game
Participants: Volunteers playing the Ultimatum Game whilst undergoing brain scanning
Procedure: The game involves two players - a Proposer offers to split money with a Responder. If the Responder accepts, money is divided as proposed; if rejected, neither player receives anything. Participants acted as Responders during fMRI scanning
Findings: When Responders rejected unfair offers (showing aggressive reactions to social provocation), brain scans revealed rapid and heightened amygdala activity. Additionally, benzodiazepine administration (which reduces autonomic nervous system arousal) decreased both rejection rates and amygdala activity
Evaluation - Strengths: Provides strong neurobiological evidence linking amygdala activity to reactive aggression using sophisticated brain imaging technology
Evaluation - Weaknesses: Laboratory setting may not reflect real-world aggression; ethical concerns about drug administration; limited to one specific type of social aggression
Serotonin
Serotonin functions as a neurotransmitter responsible for communication between neurons. It exerts widespread inhibitory effects throughout the brain, slowing and dampening neuronal activity. Normal serotonin levels in the orbitofrontal cortex correlate with reduced neuronal firing, promoting greater behavioural self-control.
Disrupted serotonin functioning may compromise this mechanism, reducing self-control and increasing impulsive behaviours including aggression. This explains why serotonin dysfunction is linked to various forms of impulsive and aggressive behaviour.
Key Study: Virkkunen et al. (1994) - Serotonin metabolites and violence
Participants: Violent impulsive and violent non-impulsive offenders
Procedure: Researchers measured levels of 5-HIAA (a serotonin breakdown product) in cerebrospinal fluid and examined sleep patterns
Findings: Impulsive offenders showed considerably lower 5-HIAA levels compared to non-impulsive violent offenders. Impulsive offenders also displayed more sleep irregularities
Evaluation - Strengths: Provides biochemical evidence for serotonin's role in aggression; sleep pattern data supports serotonin's broader regulatory functions
Evaluation - Weaknesses: Correlational design prevents causal conclusions; small sample size; potential confounding variables not controlled
Hormonal mechanisms in aggression
Testosterone
Testosterone belongs to the androgen hormone group, produced primarily in male testes with smaller amounts in female ovaries. This hormone influences masculine characteristic development and regulates social behaviour through effects on brain regions involved in aggression. The consistent observation that males display higher aggression levels than females has focused research attention on testosterone's role.
Animal research provides clear experimental evidence for testosterone-aggression links. Studies demonstrate that experimentally increasing testosterone levels correlates with greater aggressive behaviour across multiple species, whilst castration studies show the reverse pattern - reduced testosterone leading to decreased aggression.
Key Study: Dolan et al. (2001) - Testosterone and prison violence
Participants: 60 male offenders in UK maximum security hospitals
Procedure: Measured testosterone levels and assessed aggressive behaviours in individuals with personality disorders and histories of impulsive violence
Findings: Discovered a positive correlation between testosterone levels and aggressive behaviours in this high-risk population
Evaluation - Strengths: Examines real-world aggression in relevant population; large sample size for this type of research
Evaluation - Weaknesses: Correlational design; confounding variables (personality disorders, medication effects) not controlled; limited generalisability to general population
Explaining testosterone's role
Mazur (1985) developed the biosocial model of status (BMoS) to explain testosterone-aggression relationships in humans. This model proposes that testosterone levels fluctuate rapidly throughout the day, particularly responding to social interactions involving status competition. Status-related changes in testosterone should subsequently influence post-competition aggressive behaviour.
Key Study: Mehta and Josephs (2006) - Competition and testosterone
Participants: Male volunteers in competitive game situations
Procedure: Measured testosterone levels before and after competitive games (which all participants lost). After the second measurement, participants chose between challenging their victorious opponent (aggressive option) or completing an unrelated task (non-aggressive option)
Findings: Among losers whose testosterone levels increased following defeat, 73% chose to rechallenge. However, only 22% of losers with decreased testosterone levels chose to rechallenge
Evaluation - Strengths: Demonstrates temporal relationship between testosterone changes and aggressive behaviour; supports BMoS predictions
Evaluation - Weaknesses: Laboratory-based aggression may not reflect real-world violence; only examined male participants; short-term effects only
Dual-hormone hypothesis
Research evidence for testosterone-aggression links in humans remains mixed. Carré and Mehta (2011) proposed the dual-hormone hypothesis to explain these inconsistencies. They suggest that testosterone promotes aggressive behaviour only when cortisol levels remain low. When cortisol (a stress hormone central to stress responses) is elevated, testosterone's aggressive effects become blocked.
A study by Popma et al. (2007) with adolescent males confirmed this hypothesis regarding direct physical aggression, suggesting that combined testosterone and cortisol activity provides better predictions of human aggression than either hormone alone.
Evaluation
Role of other brain structures
Recent research indicates that the amygdala does not operate independently in determining aggressive behaviour. It functions collaboratively with the orbitofrontal cortex (OFC), which lies outside the limbic system. The OFC contributes to self-control, impulse regulation, and aggressive behaviour inhibition. Research by Coccaro et al. (2007) found that individuals with psychiatric disorders featuring prominent aggression showed reduced OFC activity, disrupting impulse-control functions and increasing aggressive tendencies.
Combined with Gospic et al.'s findings, this suggests that aggression regulation involves complex interactions between at least three neural structures: the amygdala, OFC, and their interconnections. This highlights the complexity of neural mechanisms in aggression.
Effects of drugs on serotonin
Drugs that enhance serotonin activity also reduce aggressive behaviour levels, providing additional evidence for serotonin-aggression links beyond correlational findings.
Key Study: Berman et al. (2009) - Paroxetine and aggression
Participants: Individuals with and without prior aggressive behaviour history
Procedure: Administered either placebo or paroxetine (a serotonin-enhancing drug) before participation in a laboratory aggression task involving electric shock administration in response to provocation
Findings: Paroxetine participants consistently delivered fewer and less intense shocks compared to placebo participants, but only among those with previous aggressive behaviour histories
Evaluation - Strengths: Experimental manipulation provides stronger causal evidence than correlational studies; controlled conditions
Evaluation - Weaknesses: Laboratory aggression may not reflect real-world violence; ethical concerns about electric shock administration; effects limited to those with aggressive histories
Issues of cause and effect
Most neural and hormonal aggression research employs correlational methods due to ethical constraints preventing experimental manipulation of brain structures and hormones in humans. However, correlational evidence cannot establish causal relationships or rule out third variable influences.
This limitation means that neural and hormonal aggression regulation is likely more complex than current understanding suggests. Research demonstrating interactions between testosterone and cortisol (Carré & Mehta, 2011) supports this complexity argument. Alternative approaches involve experimental manipulation studies, which are ethically acceptable with animals (such as male rat castration studies) but raise generalisability questions regarding human aggression.
Key Points to Remember:
-
Neural mechanisms: The limbic system, particularly the amygdala, plays a key role in threat assessment and aggressive responses, while serotonin acts as an inhibitory neurotransmitter that reduces impulsive behaviour when functioning normally.
-
Hormonal influences: Testosterone is associated with increased aggression, especially in males, but its effects may depend on interactions with other hormones like cortisol according to the dual-hormone hypothesis.
-
Complex interactions: Aggression involves multiple brain structures working together, including the amygdala and orbitofrontal cortex, rather than single isolated mechanisms.
-
Research limitations: Most human studies are correlational due to ethical constraints, making it difficult to establish definitive causal relationships between biological factors and aggressive behaviour.
-
Practical applications: Understanding these mechanisms has led to drug treatments that can reduce aggressive behaviour by enhancing serotonin function, though effects may be limited to individuals with existing aggressive tendencies.