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Predicting the onset of mental illness could soon be as simple as smelling a scratch-and-sniff card loaded with the aroma of roses or a whiff of petrol.

Scientists have taken the same technology popular in children's books and designed a test to help diagnose brain disorders before the onset of any symptoms.

The test can be used for Alzheimer's and Parkinson's diseases, obsessive-compulsive disorder and schizophrenia, as well as some illnesses affecting adolescents.

It originated in a discovery by Melbourne University researchers of a link between these illnesses and a poor ability to identify smells.

To test their theory, they developed a set of 40 scratch-and-sniff cards and asked people to identify the smell from a list of four possibilities, such as coffee, roses, oranges and petrol.

Professor Warwick Brewer, from the university's Orygen Research Centre, said the people who later went on to develop a brain disorder had demonstrated difficulty correctly answering more than half the questions.

He said the simple test also could be used by relatives of people with these conditions.

"Because of the genetic link in many illnesses, it is hoped the test could also be used by family members of people who have developed an illness of the brain."

Professor Christos Pantelis, from the Melbourne Neuropsychiatry Centre, said smell ability provided unique information about brain structure and function.

"Mental illness can arrest the full maturation of the frontal lobe, while degenerative illness can damage it," Professor Pantelis said.

"This area of the brain is used to analyse and identify smells so an abnormal sense of smell may indicate problems in this 'thinking' area of the brain."

Their research also revealed that the sense of smell is worse in those with more severe illnesses, giving important clues into the patient's long-term prospects.

The research has been compiled in a new book, Olfaction and the Brain.