Science for Everybody

Contents

• Introduction
• Genetic Explanations of Offending Behaviour
• Twin Studies
• Adoption Studies
• Candidate Genes: MAOA and CDH13
• The Diathesis-Stress Model
• Neural Explanations of Offending Behaviour
• The Prefrontal Cortex
• Neurotransmitters and Serotonin
• Linking Genes and Neural Functioning
• Real-World Significance
• Evaluation
• Strengths
• Weaknesses
• Summary
• Quick Questions
• Key Terms

---

Introduction

Biological explanations argue that some people may have inherited or brain-based characteristics that increase the risk of offending behaviour. Genetics can influence traits such as impulsivity and aggression, while neural factors may affect emotion, decision-making and self-control. However, biological risk does not mean that offending is inevitable, so interaction with the environment remains important.

This content matches the AQA A-level Psychology Forensic Psychology specification area on biological explanations of offending behaviour: genetic and neural explanations.

---

Genetic Explanations of Offending Behaviour

Genetic explanations suggest that people may inherit biological characteristics that increase vulnerability to offending. Modern psychology does not support the idea of a single “criminal gene”. Instead, researchers usually treat offending as a complex, polygenic outcome in which many genes each make a small contribution and interact with environmental experiences.

Twin Studies

Twin studies compare concordance for offending between monozygotic (MZ) twins and dizygotic (DZ) twins. If MZ twins show higher concordance than DZ twins, researchers infer that genes may contribute because MZ twins share more genetic material.

However, MZ twins can also experience more similar environments. Therefore, higher concordance cannot prove a genetic cause by itself.

Lange (1930)

Aim: To investigate whether criminal behaviour showed a stronger similarity in identical twins than in non-identical twins.

Procedure/method: Lange examined male twin pairs in which at least one twin had been imprisoned. The sample included 13 MZ pairs and 17 DZ pairs, classified using the methods available at the time.

Findings: In 10 of the 13 MZ pairs, the co-twin had also been imprisoned, compared with only 2 of the 17 DZ pairs.

Conclusion: The much higher concordance among MZ twins suggested that inherited factors may contribute to offending behaviour. However, the design could not rule out shared environmental influences.

Adoption Studies

Adoption studies offer another way to separate heredity from environment. If adoptees resemble their biological relatives in offending despite being raised by different parents, this supports a genetic influence.

In contrast, similarity with adoptive relatives suggests that environmental learning and family context also matter.

Crowe (1972)

Aim: To test whether having a biological parent with a criminal record increased the later risk of criminality in adopted children.

Procedure/method: Crowe compared adopted children whose biological mothers had criminal records with adopted children whose biological mothers did not. Because the children grew up away from their biological mothers, the design reduced direct environmental transmission.

Findings: The group with a biological parent who had a criminal record showed a much higher rate of later criminal records. Textbook summaries commonly report around 50% in the higher-risk group compared with about 5% where the biological mother had no criminal record.

Conclusion: The findings support a genetic contribution to offending. However, they do not show that genes act independently of prenatal conditions, adoption placement or later environment.

Candidate Genes: MAOA and CDH13

Candidate-gene research looks for specific genetic variants that occur more often in groups with particular behavioural characteristics.

Two genes often discussed in forensic psychology are MAOA and CDH13.

MAOA affects an enzyme involved in breaking down monoamine neurotransmitters. CDH13 contributes to neural development and cell signalling.

Neither gene causes crime. Instead, particular variants may alter vulnerability to traits associated with some forms of violent or impulsive behaviour.

Tiihonen et al. (2014)

Aim: To identify genetic variants associated with severe violent offending in a Finnish offender sample.

Procedure/method: The researchers analysed genetic data from almost 900 Finnish offenders and compared patterns in violent and non-violent groups. They focused particularly on variants involving MAOA and CDH13.

Findings: Variants of MAOA and CDH13 occurred more often among the most violent offenders. The high-risk combination was strongly overrepresented, with textbook accounts reporting that carriers were about 13 times more likely to have a history of severe violent behaviour than comparison groups.

Conclusion: Specific genetic variants may contribute to vulnerability to extreme violent behaviour. However, the researchers did not establish a single genetic cause, and the findings require cautious interpretation and replication across populations.

The Diathesis-Stress Model

A stronger modern explanation combines biological vulnerability with environmental risk.

In a diathesis-stress account, the diathesis is an inherited or biological predisposition. Stress can include:

• adverse childhood experiences
• inconsistent parenting
• exposure to violence
• substance misuse
• deprivation
• antisocial peer groups

Offending becomes more likely when vulnerability and environmental risk combine.

Mednick et al. (1984)

Aim: To examine how biological and adoptive parental criminality related to convictions in adopted children.

Procedure/method: The researchers studied more than 14,000 Danish adoptees and compared conviction rates according to whether their biological parents, adoptive parents, both sets of parents or neither had criminal convictions.

Findings: When neither biological nor adoptive parents had convictions, about 13.5% of adoptees had convictions. The rate rose to about 20% when biological parents had convictions, about 14.7% when only adoptive parents had convictions, and about 24.5% when both biological and adoptive parents had convictions.

Conclusion: The pattern supports both genetic and environmental influences. The highest risk occurred when biological vulnerability and an adverse family environment appeared together, which fits a diathesis-stress explanation.

A useful way to interpret genetic findings is probabilistically. Genes may change the likelihood of traits such as impulsivity, emotional reactivity or poor behavioural control, but they do not write a fixed criminal outcome.

Many people who carry proposed risk variants never offend, and many offenders do not carry those variants.

---

Neural Explanations of Offending Behaviour

Neural explanations focus on differences in brain structure, brain functioning and neurotransmitter activity.

Researchers have paid particular attention to systems involved in:

• impulse control
• planning
• empathy
• reward
• emotional regulation

Again, these differences represent possible risk factors rather than a biological label for a “criminal brain”.

The Prefrontal Cortex

The prefrontal cortex helps people inhibit impulses, consider consequences, plan behaviour and regulate emotional responses.

Reduced functioning in this area could make it harder to suppress aggressive reactions or evaluate long-term consequences.

Therefore, it may increase risk in some individuals, especially when combined with provocation, substance use or other environmental pressures.

Raine et al. (1997)

Aim: To investigate whether people charged with murder who had pleaded not guilty by reason of insanity showed different patterns of brain activity from controls.

Procedure/method: Raine and colleagues used PET scans with 41 murderers and 41 matched controls. Participants completed a continuous performance task while the researchers measured glucose metabolism across different brain regions.

Findings: The murderer group showed reduced activity in the prefrontal cortex and atypical activity in several limbic and subcortical regions involved in emotion and behaviour regulation.

Conclusion: Abnormal functioning in neural systems linked with inhibition, emotion and decision-making may be associated with serious violent behaviour. However, the correlational design cannot establish whether the brain differences caused the offending.

Related structural research has also reported reduced prefrontal grey matter in some people with antisocial personality traits.

This fits the idea that weaker executive control may contribute to impulsive or poorly regulated behaviour. However, brain structure changes through development and experience, so researchers must avoid treating these differences as fixed causes.

Neurotransmitters and Serotonin

Serotonin helps regulate mood, inhibition and emotional control.

Low serotonergic functioning has been associated with impulsive aggression in some research. One proposed mechanism suggests that weak serotonin regulation reduces the ability to inhibit immediate emotional reactions.

As a result, a person may respond more aggressively to frustration or threat.

However, serotonin does not map neatly onto criminal behaviour. Social context, personality, alcohol or drug use, learning history and situational triggers can all change how aggression appears.

Linking Genes and Neural Functioning

Genetic and neural explanations overlap.

For example, MAOA influences the metabolism of monoamine neurotransmitters. Therefore, a genetic variant can affect neural signalling.

It can be misleading to treat “genetic” and “neural” explanations as completely separate. A more complete account traces a pathway from genes to brain development and neurotransmitter regulation, then considers how the environment shapes whether those vulnerabilities become behaviour.

Reflection question: If a biological characteristic raises the statistical risk of impulsive violence but most people with that characteristic never offend, how should psychologists, courts and society use that information without treating risk as destiny?

Real-World Significance

Biological evidence has sometimes appeared in legal arguments about responsibility and sentencing.

The Mobley case in the United States is often used as an example. The defence sought genetic testing to support a claim that violent behaviour may have had a biological basis.

Courts have generally remained cautious because genetic and neural findings describe probabilities across groups rather than proving why one individual committed one offence.

The strongest practical use of biological research may therefore lie in understanding risk, tailoring rehabilitation and improving prevention rather than excusing behaviour.

---

Evaluation

Strengths

Converging Evidence

Converging evidence strengthens the biological account. Twin studies, adoption studies, molecular genetics and neuroimaging use different methods, yet each has found some association between biological factors and offending-related outcomes. For example, Lange reported higher concordance in MZ than DZ twins, while Mednick et al. found higher conviction rates when biological parents had convictions. Because different methods point in a similar direction, the argument that biology contributes to offending has greater credibility than any single study alone.

Interactionist Explanation

The diathesis-stress approach avoids a simple nature-versus-nurture explanation. Mednick et al. found the highest conviction rate when both biological and adoptive risk were present. This pattern suggests that genetic vulnerability may combine with environmental conditions rather than acting alone. Therefore, an interactionist account explains why many biologically vulnerable people do not offend and why prevention can still matter.

Testable Neural Mechanisms

Neural research can generate useful, testable mechanisms. Findings involving the prefrontal cortex connect offending risk with psychological processes such as inhibition, planning and emotional regulation. Researchers can test these hypotheses using brain imaging, cognitive tasks and longitudinal research. As a result, neural explanations offer more precise predictions than vague claims that offenders are simply “biologically different”.

Practical Applications

Biological research may support practical intervention when used carefully. If impulsivity, poor emotional regulation or neurodevelopmental difficulties increase risk, services can focus on self-control training, substance misuse treatment, early family support and rehabilitation. The practical value lies in reducing risk, not in using biology to label people as future offenders.

Weaknesses

Problems with Twin Studies

Twin studies struggle to separate shared genes from shared environments. MZ twins often look more alike, receive more similar treatment and may spend more time together than DZ twins. Therefore, higher MZ concordance may partly reflect environmental similarity rather than genetic influence. Early research such as Lange also relied on small samples and older methods of classifying zygosity. This reduces validity and temporal validity.

Problems with Adoption Studies

Adoption studies do not create a perfectly clean split between genes and environment. Adoption agencies may use selective placement, meaning adoptive homes can resemble biological homes in social class or other characteristics. Prenatal influences also remain with the child, and some adoptees retain contact with biological relatives. Consequently, apparent genetic effects may include environmental influences researchers cannot fully control.

Problems with Candidate-Gene Findings

Candidate-gene findings can be difficult to replicate. Serious offending is likely to be highly polygenic, with many variants making tiny contributions. Small samples, population differences and multiple statistical comparisons can produce associations that fail to generalise. Tiihonen et al. used a Finnish sample, so population-specific genetic structure and sample characteristics may limit cultural generalisability. Modern genetics therefore treats simple single-gene explanations with caution.

Correlation and Causation

Neuroimaging research usually shows correlation rather than causation. Raine et al. found reduced prefrontal activity in a group of murderers, but the scans cannot show whether those neural differences existed before offending. Head injury, chronic stress, substance misuse, medication or life experiences could alter brain functioning. Therefore, claims that brain abnormalities cause crime go beyond the evidence.

Sample Bias

Sample bias limits many biological studies of offending. Researchers often recruit male prisoners, violent offenders or people with severe antisocial characteristics because they are easier to define and access. Findings may not apply to women, young offenders, people convicted of non-violent crimes or people who offend without being detected.

Reductionism and Determinism

Biological explanations risk reductionism and determinism. Reducing crime to genes, neurotransmitters or brain regions can ignore poverty, peer influence, social learning, trauma, opportunity and cultural norms. Deterministic interpretations can also affect legal responsibility and create stigma. A genetic or neural risk factor may explain part of vulnerability, but it does not remove personal agency or prove that someone would inevitably offend.

Cultural Bias and Temporal Validity

Cultural bias and temporal validity also matter. Definitions of crime change across cultures and historical periods, while police practices affect who receives convictions. A biological study that uses conviction records may therefore partly measure social processes such as policing and sentencing. Older studies may also reflect outdated diagnostic methods, family structures and legal systems.

Ethical Concerns

Ethical concerns arise when researchers connect biology with criminality. Genetic screening could encourage discrimination in employment, insurance or criminal justice. Brain-based labels could also encourage attempts to predict dangerousness before someone has offended. Psychologists should therefore communicate risk as probabilistic, protect confidentiality and avoid turning group-level associations into judgements about individuals.

Quick Questions

What is the difference between monozygotic and dizygotic twins in genetic similarity?

MZ twins share virtually 100% of their genes, while DZ twins share about 50% of their segregating genes on average.

What did Lange (1930) find about offending concordance in MZ and DZ twins?

Lange reported that 10 of 13 MZ co-twins had also been imprisoned, compared with 2 of 17 DZ co-twins.

Which two candidate genes did Tiihonen et al. (2014) link with severe violent offending?

MAOA and CDH13.

What does the diathesis-stress model suggest about biological vulnerability and the environment?

An inherited or biological vulnerability may increase the risk of offending, but environmental stressors help determine whether that vulnerability develops into behaviour.

Why might reduced prefrontal cortex functioning increase the risk of impulsive offending?

The prefrontal cortex supports inhibition, planning and consideration of consequences, so reduced functioning may weaken behavioural control.

Key Terms

Genetics – The study of inherited biological information and how genes influence behaviour.

Candidate gene – A gene selected for investigation because researchers think it may contribute to a behaviour or trait.

MAOA – A gene that affects the enzyme monoamine oxidase A, which helps regulate neurotransmitters including serotonin, dopamine and noradrenaline.

CDH13 – A gene involved in neural development and cell signalling that has been investigated in relation to violent offending.

Twin study – A method that compares similarities between monozygotic and dizygotic twins to estimate genetic influence.

Monozygotic (MZ) twins – Identical twins who share virtually 100% of their genes.

Dizygotic (DZ) twins – Non-identical twins who share, on average, about 50% of their segregating genes.

Concordance rate – The degree to which both members of a pair show the same characteristic or behaviour.

Adoption study – A method that compares adoptees with biological and adoptive relatives to separate genetic and environmental influences.

Diathesis-stress model – The view that an inherited vulnerability may lead to offending only when environmental stressors or risk factors activate it.

Neural explanation – An explanation that links behaviour to brain structures, brain functioning or neurotransmitters.

Prefrontal cortex – The front part of the cerebral cortex involved in planning, decision-making, inhibition and regulation of behaviour.

Serotonin – A neurotransmitter involved in mood, impulse control and emotional regulation; unusually low functioning has been linked with impulsive aggression.

Biological determinism – The view that biological factors strongly or completely determine behaviour.

Biological reductionism – Explaining complex behaviour using a narrow set of biological processes while underplaying social and psychological influences.