LONDON (Reuters) - Children born to older fathers have a higher than normal risk of developing schizophrenia later in life, Swedish scientists said on Friday.

They suspect that accumulating mutations in the sperm of older men add to the risk of their children suffering from the psychotic disorder.

"There is an association between paternal age and the risk of schizophrenia in the offspring," Professor Finn Rasmussen, of the Karolinska Institute in Stockholm, told Reuters.

Other researchers have suggested such a link before but Rasmussen said his study of 700,000 people in Sweden is the largest and most significant.

"On a population level this is important because of the increasing paternal age of the population," he added. Career demands and other factors have resulted in couples postponing having children. In England and Wales the average paternal age has increased from 29.2 in 1980 to 32.1 in 2002.

Rasmussen and his team, who reported their research in the British Medical Journal, estimated that the three-year age increase in the last 20 years could result in 12-15 percent more cases of schizophrenia.

The causes of schizophrenia are unknown. Scientists believe it affects chemicals in the brain and that there is a biological link which can predispose a person to the disease.

It affects about 24 million people worldwide, mostly in the 15-35 age group. Although it is a treatable illness, more than 50 percent of sufferers do not receive appropriate care, according to the World Health Organisation.

Schizophrenia usually begins in the late teens and early 20s and is characterised by hallucinations, delusions, hearing voices and changes in behaviour.

In their study of people born between 1973 and 1980, Rasmussen and his colleagues analysed hospital admissions for schizophrenia and other disorders between 1989 and 2001.

They calculated that, overall, 15.5 percent of schizophrenia cases in the study group may have been due to the patient having a father who was over 30 years at the birth.

The risk increases with age, so a child born to a man of 50 could have a higher risk than one whose father was 30.

"The occurrence of mutation in sperm increases with age," Rasmussen added.

SOURCE: British Medical Journal, October 23, 2004.

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