Genetic explanations argue that some people inherit a biological vulnerability to offending. Researchers look for patterns of criminal behavior in families and for genes associated with aggression, impulsivity, and antisocial tendencies.

Genetic explanations of offending behavior

Genetic accounts suggest that offending is partly influenced by inherited characteristics. This does not mean there is a single “crime gene” or that genes directly cause criminal acts. Instead, the modern view is that offending is usually polygenic, meaning many genes each make a small contribution to risk.

Genes may affect traits linked to offending, such as impulsivity, aggression, low self-control, or a tendency to seek excitement. These inherited tendencies can increase the likelihood of offending, especially when combined with environmental risk factors such as poor parenting, abuse, or deprivation. Genetic explanations are therefore usually probabilistic, not deterministic: they suggest a greater chance of offending, not certainty.

Evidence from family, twin, and adoption research

One way to test genetic explanations is to see whether offending runs in families. If criminal behavior is more common among biological relatives, this may suggest an inherited influence. However, family studies alone are weak evidence because relatives often share the same environment as well as the same genes.

Twin and adoption studies are more useful because they help separate genetic similarity from environmental similarity.

Standard ACE twin-model diagram illustrating how variation in a trait can be decomposed into additive genetic influences (A), shared environmental influences (C), and non-shared environmental influences (E). It visually encodes the core logic of twin studies: MZ twins correlate 1.0 genetically, DZ twins 0.5, while shared environment is assumed to be 1.0 for both. Source

Twin studies

Twin studies compare monozygotic (MZ) twins, who share 100 percent of their genes, with dizygotic (DZ) twins, who share about 50 percent on average.

Diagram showing how monozygotic (identical) twins arise from one fertilized egg that splits, whereas dizygotic (fraternal) twins arise from two separate fertilized eggs. This supports the key methodological assumption in twin studies: MZ pairs share (approximately) all their genes, while DZ pairs share about half on average. Source

If MZ twins are more alike in offending than DZ twins, this supports a genetic explanation.

Researchers often compare twins using concordance rates.

A classic study by Christiansen (1977) examined over 3,500 twin pairs in Denmark. He found that the concordance rate for criminal convictions was much higher for MZ twins than for DZ twins, with figures of around 52 percent for MZ twins and about 22 percent for DZ twins. This suggests that genes contribute to offending behavior.

At the same time, the MZ concordance rate was far below 100 percent. This matters because if genes were the only cause, both twins in every MZ pair would offend. The findings therefore imply that environmental influences also matter.

Adoption studies

Adoption studies look at whether adopted children resemble their biological parents or their adoptive parents in criminal behavior. If adoptees are more likely to offend when their biological parent has a criminal record, this supports genetic influence, because they do not share the same home environment.

A well-known study by Mednick et al. (1984) investigated thousands of adopted boys in Denmark. They found that adoptees were more likely to have a criminal conviction if their biological father had one. The risk was even higher when both the biological and adoptive fathers had convictions. This supports the idea that offending may involve both genetic vulnerability and environmental risk.

Adoption research is valuable because it reduces the problem of shared family environment, but it still does not prove that genes act alone.

Candidate genes and molecular genetics

More recent research tries to identify particular genes that may contribute to offending.

One widely discussed example is the MAOA gene, sometimes linked with aggressive and antisocial behavior. MAOA affects the activity of enzymes involved in breaking down neurotransmitters. Some variants of this gene have been associated with higher levels of aggression, particularly in people who experienced childhood maltreatment. This suggests that genes may create a vulnerability, but whether that vulnerability is expressed may depend on the environment.

Norm-of-reaction graph demonstrating gene–environment interaction: different genotypes show different outcomes across environmental conditions (non-parallel lines). This is the key idea behind claims that certain genetic variants (e.g., low-activity MAOA) may increase risk mainly under environmental adversity rather than causing offending on their own. Source

Researchers have also argued that offending is not caused by one gene alone. It is more likely to be:

• polygenic: many genes contribute small effects

• interactive: genes work alongside life experiences

• complex: different genes may relate to different forms of offending, such as violent or impulsive crime

This makes simple one-gene explanations too limited for most real criminal behavior.

Evaluation of genetic explanations

Genetic explanations have several strengths.

• They are supported by scientific evidence from twin, adoption, and molecular genetic studies.

• Findings are often based on objective measures, such as official conviction data.

• They explain why offending sometimes appears to cluster in families, even when children are raised apart from biological parents.

However, there are important limitations.

Correlation does not prove causation

Even strong associations between genes and offending do not show that genes directly cause crime. A person may inherit a tendency toward aggression or impulsivity, but whether this becomes offending depends on other influences.

Twin studies may overestimate genetic effects

MZ twins are often treated more similarly than DZ twins, so higher concordance may partly reflect environmental similarity, not just genetic similarity. This is called the equal environments assumption problem.

Adoption studies are not perfect

Adopted children may still experience environments that are not typical, and some are placed with families similar to their biological background. This can blur the distinction between genetic and environmental influence.

Criminal convictions are an incomplete measure

Many studies use official convictions, but not all offenders are caught or convicted. This means researchers may be measuring patterns in detection and sentencing as well as actual offending.

Genetic explanations can be reductionist and deterministic

A purely genetic account risks reducing criminal behavior to biology and ignoring social context, learning, and personal choice. It can also encourage biological determinism, the idea that people are “born criminals.” In reality, the evidence shows that genes usually create risk, not destiny.

Gene-environment interaction is crucial

The strongest modern view is that genes may predispose a person to offend, but environmental stressors often determine whether that predisposition is expressed. This means genetic explanations are useful, but incomplete on their own.