Although old people account for around 1% of arrests, the number of elderly offenders is increasing at a disproportionate rate, according to psychiatrist Dr Graeme Yorston.

And it is expected to rise still further with the predicted explosion in elderly people in the next century.

Dr Yorston, who presented the results of a two-year study into elderly offenders to a Royal College of Psychiatrists meeting last week, says that research in this area is very limited.

What literature there is shows older people suffer much higher levels of psychiatric illness than other offenders.

"Up to 50% of over 60s charged with a crime may suffer from a mental illness," he said.

Alcoholism is one of the most common illnesses, but it depends on the crime.

Sex offenders who had a mental disorder were more likely to suffer from dementia, depression or personality disorder, said Dr Yorston.

Shoplifters tended to suffer from depression, as did the very small proportion of elderly homicides with mental illness.

Frontal lobe dementia

Dr Yorston, who works at Lanarkshire Healthcare NHS Trust, said elderly sex offenders may have a form of dementia called frontal lobe dementia.

This affects their decision-making.

"They become reckless and sexually disinhibited, for example, exposing themselves and approaching small children," said Dr Yorston.

He added that mentally ill elderly people were likely to be at the milder end of sex offending and to have no previous record of offending.

Old people are much more likely to be victims of crimes than perpetrators

"The stereotype of elderly sex offenders is that they are dirty old men. Some are. They tend to be people who have offended throughout their lives and it is right and proper that the criminal justice system should deal with them," said Dr Yorston.

"But some are not bad. Their brain has been affected and they cannot help themselves. They need psychiatric care rather than jail."

He added that elderly offenders would tend, if assessed by psychiatrists, to be examined by forensic psychiatrists.

"But many may have little experience of treating the elderly and may miss a diagnosis," he said.

Divisions

He believes psychiatric specialisms are too divided and that forensic and elderly experts need to communicate more.

He also says more services need to be developed to help elderly offenders.

The US has several elderly inmate units and court diversion schemes to support the elderly.

But the UK has just one unit in Portsmouth which is for elderly lifers and was set up by local doctors.

Dr Yorston said the UK had schemes to support young offenders.

These mean mentally ill offenders get medical help instead of having to go to court.

"What tends to happen with elderly people is that police turn a blind eye unless their crime is serious and they miss out on getting any help," said Dr Yorston.

Another problem is the shortage of psychiatrist specialising in the elderly.

Dr Yorston says it is an attractive specialism since it has not been as bogged down with legislation as other areas of psychiatry in the last 10 years.