A further study type when looking at physiological affects is to look at twin studies. Twin studies use concordance rates to compare twin’s behaviour. They look at sets of twins and compare whether or not both display the same behaviour. Identical twins (Monozygotic or MZ) share 100% of their genes and non-identical (Dizygotic or DZ) twins share 50% of their genes so it is expected that MZ twins will have a higher concordance rate. Many studies have looked at concordance rates for twins and criminal behaviour.
Hollin (1989) said that there could be a genetic factor involved in criminality after reviewing many twin studies and finding 48% concordance rate for criminality of MZ twins and only a 20% concordance rate for DZ twins. Twin studies, though, are problematic since MZ twins and DZ twins share the same home environment. Furthermore, it has been suggested that MZ twins are treated even more alike since they look so similar, and receive a more equal environment that DZ twins, who look different.
So the nature/nurture debate is not well addressed by this type of study. At one time it was suggested that a condition known as XXY syndrome could be responsible for criminality, and particular violent crimes, when Sandberg et al (1961) found that 5% of prisoners were found to have the syndrome. However, this was at a time when XXY syndrome had only just been discovered and little was known about it. It was less well known then that people from all walks of life could be found to have the syndrome.
This idea was hence criticised when Owen found in 1972 that there were not more XXY males in prison than in the general population. He also found out that those in prison with the syndrome were more likely to have committed a sex offence as opposed to a violent crime. More minor studies have attempted to show a correlation between ADHD (Attention Deficit Hyperactivity Disorder) and delinquency as well as diet and criminality – though this can be said to be down to class and criminality, since different classes can be shown to have different diets.
Some studies have also turned up inconclusive results, such as Loomis et al’s study in 1967 that found no correlation between EEG (electroencephalogram) patterns and criminal behaviour. A further approach to physiology affecting behaviour has been to look at the effect of brain damage or dysfunction. Mitchell and Blair (1999) looked at how the amygdala affects psychopaths and found that a dysfunctioning amygdala means that people are less aware of empathy and don’t know when to stop being aggressive – leading to violent crimes.
A study by Raine, Buchsbaum & Lacasse (1197) showed how many criminals had different brain activity compared to their controls. It found that most of the brain activity was lower than their controls leading to less control over the violence that they display. One part of their brain, the Corpus Callosum, was showing less activity in criminals than controls, which may stop the left side of the brain inhibiting the rights side’s violence. Their study is however criticised for having small sample that was mainly men and only covered a small age range, meaning that it could be less generalised to all criminals.
In conclusion, it has been found that whilst it appears that genetics and biological factors can lead to criminality they cannot be said to be the sole reason for it. Many studies appear to show that it is physiological reasons that lead to criminality but these cannot be shown to be separate from environmental factors, so they cannot be said to cause it alone. The extent that physiological factors explain why people become criminals is quite small. It does show that genetic predispositions can lead to a greater chance of criminality but they cannot be said to be the only reason for it.
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