Crime may be an unusual topic for a medical column but is a growing area of scientific research.

The various contributions of biological factors or "nature" versus the social environment, "nurture", is hotly debated. New brain scanning techniques and quick and affordable genetic testing is rapidly improving our understanding of the science behind crime.

The brains of people who undertake serious or sexual crimes seem to differ in a number of ways.

A controversial study from Yale University used MRI scans to compare the brains of paedophiles and those convicted of non-sexual crimes. Paedophiles had significantly less of a substance called "white matter" that connects six areas of the brain known to play a role in sexual arousal.

The researchers' theory was that the lack of adequate wiring between these centres resulted in paedophiles being unable to differentiate between appropriate and inappropriate sexual objects.

New brain scanning called Pet (positron emission tomography) also allows the activity of the brain to be examined by measuring the uptake of glucose, the "fuel" of the brain. Pet has demonstrated that a region at the front of the brain - the prefrontal cortex - functions differently in murderers. This area normally controls the behaviour that makes us "civilised" - self-control, maturity, judgment, tactfulness and reasoning.

Violent crimes have also been associated with less specific abnormalities in the frontal lobe.

Hormones are chemical messengers that travel in the bloodstream. Studies show many females imprisoned for aggressive criminal acts commit their crimes during the premenstrual phase of their menstrual cycle. This suggests that fluctuations in female sex hormones levels may regulate impulsive behaviour.

Further, recent research demonstrated that male adolescents with severe anti-social behaviour don't produce as much of the stress hormone cortisol in response to high-tension situations.

Fairchild and colleagues, publishing in the journal Biological Psychiatry last month, speculate this may lead to less caution and more anger and impulsiveness during times of stress. The genetics behind criminal behaviour are contentious. Whether having a certain combination of genes will make you a criminal is open to debate. If criminal behaviour is pre-determined, genetics could be used as a legal defence.

The concept of genetic criminology dates from the 19th and early 20th centuries, when prominent researchers believed genes could be fully responsible for criminal activity. As a result, and in line with a growing eugenics movement, acts of sterilisation took place to rid society of future generations of "criminals, idiots, imbeciles, and rapists".

During the 1960s, studies of prison inmates provided further evidence. They found men who carried an extra Y chromosome were more likely to end up in prison. While this was widely publicised at the time, more recent research has cast doubt on these original findings.

However, further support of the gene theory has since emerged as a result of the human genome project and subsequent rapid throughput genetic screening. A growing number of genes have recently been linked with antisocial behaviour and violent psychiatric conditions. Not surprisingly, many of these genes have been found to control chemical signals in the brain and hormone levels.

While environmental factors are undoubtedly important, there is clearly a biological contribution to criminal behaviour. New technology is improving understanding. United States court rooms have recently allowed defence attorneys to use MRI scans as evidence of a defendant's predisposition to criminal behaviour. Whether genetic profiles or brain scans will ever prove to be an acceptable and ethical defence in a UK court of law remains to be seen.