Genetic Theories of Crime


XYY Theory

This theory suggests that some crime might be attributable to a chromosomal abnormality. Chromosomes are structures in cell nuclei and humans usually have 46 chromosomes, 44 of which determine the shape and constitution of our body, and 2 determining sex.

There are a variety of chromosomal abnormalities, some of which involve the presence of extra chromosomes, one such condition, known as ‘XYY’ involves the presence of an extra Y chromosome.

XYY men, sometimes called super males, have been of interest to criminologists because of the suggestion that they are more aggressive and more inclined to be violent than males with a single Y chromosome.

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Jacobs et al. (1965) suggested that men with the XXY chromosome were more aggressive than normal ‘XY’ men. Patricia Jacobs in 1965 conducted a study of inmates in a Scottish maximum security hospital. The subjects of this study were defined as “mentally subnormal male patients with dangerous, violent, or criminal propensities”

Jacobs found that 1 in 28 of these subjects were XYY. This high rate compared  with the incidence in the general population, seemed to suggest that the XYY condition could indeed be linked to increased violent or anti-social behaviour. 

The serial killer John Wayne Gacy (‘The Killer Clown’) is said to have XYY syndrome. He sexually assaulted and killed at least 33 men in the USA.

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Jacob et al found they were overrepresented in the prison population.

Adler et al (2007) argued that it is possible that aggressive and violent behaviour is at least partly determined by genetic factors.


Not all men with XYY commit crime or act aggressively.

The theory ignores environmental factors i.e. upbringing. Having XYY can make you appear different which could lead to bullying or marginalization.

Theilgaard (1984) found correlation between XYY and criminals but doesn’t prove causation.

Theory can’t explain all men who show aggression or commit crime. Also can’t explain female criminality as its only a gene found in men.

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Adoption Studies

Researchers have also used adoption studies to test for a genetic cause of crime. These studies compare children both to their biological birth parents and to their adopted parents.

The thinking behind adoption studies is that an adopted child (esp. if adopted soon after birth) shares the same environment as their adoptive parents, but the same genes as their biological parents.

If we find that the adoptee’s behaviour in regard to criminality is more similar to their birth parents’ behaviour, this would support a genetic explanation.

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Mednick et al examined data on over 14,000 adopted sons in Denmark from 1924 to 1947. They found that sons were more likely to have a criminal record if a birth parent also had a record (20% probability rate). This supports a genetic explanation. By contrast, they found that a smaller proportion (14.7%) had a criminal record if their adoptive parent had one.

Hutchings and Mednick compared adoptees with and without criminal records. They found that adoptees with criminal records were more likely to have biological parents with criminal records than adoptees whose birth parents did not have criminal records.

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In adoption studies individuals are exposed to a different environment to their biological or genetic relatives. This makes it easier to separate genetic and environmental factors.

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Generalisability. Adoption studies use adopted children which isn't very representative of the general population of children. Some of these children may have also gone through trauma which would affect the results as they may behave differently to most children. This means results can't be generalised to a general population of children.

Reliability-. Results aren’t always reliable because the studies are opportunistic which means they use what they can find at the time. This is an issue as some other variables may affect children's behaviour I.e. the children may have spent some time as orphans and this early upbringing might affect them.

Ethics. Children cannot consent to be studied, but adoption studies usually proceed with the presumptive consent of the adoptive parents and (where possible) the biological parents too. The children's anonymity is preserved, and this respects their privacy and dignity.

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Twin Studies

Genetic theories have used studies of identical twins as a way to test their theory of criminality. This is because identical or monozygotic (MZ) twins share exactly the same genes – they both developed from the same fertilised egg. Therefore if one twin is criminal, the other twin ought to be criminal too.

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Evidence for this comes from Christiansen’s study of 3,586 twin pairs in Denmark.

He found that there was a 52% concordance (probability) rate between MZ twins; that is where one identical twin had a conviction, there was a 52% chance of the other twin also having a conviction. But among non-identical (dyzgotic or DZ) twins, there was only a 22% chance.

A similar study by Ishikawa and Raine found a 44% concordance rate for identical twins but only 21.6% for non-identical twins.

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Twin studies can tell us whether important behaviours are heritable - which means they are passed down genetically from your parents rather than learned from your environment.
Reliability. Because of fertility treatment and women having babies later in life, the number of multiple births is increasing. This increases the number of twins available for study, making it possible to conduct research with large samples, analyse data with more powerful statistical tests and making it easier for other psychologists to replicate studies.
Validity. Twins are an example of a naturally-occurring variable being perfectly changed for study: MZ twins share 100% of their genes, DZ twins share about 50%. Because researchers are not manipulating this variable themselves, it reduces the risk of researcher bias.

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Twins are rarely separated at birth and raised in a totally different environment, yet this is necessary to substantiate claims. MZ twins reared together share many of the same experiences. DZ twins reared together may not share the same experiences due to levels of attractiveness or temperament.

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