Dementia breaks down the brain’s ability to perform complex tasks, like this one.

Being unable to understand sarcasm is an early warning sign of dementia, research finds.

Deterioration in key parts of the brain in neuro-degenerative diseases causes people to have problems spotting insincere communication.

Detecting lies and sarcasm is a relatively complex cognitive task.

It requires being able to simulate the other person’s mind and inferring what they must mean.

Those with dementia also find it hard to spot lies.

Dr Katherine Rankin, who led the study, said:

“These patients cannot detect lies.

This fact can help them be diagnosed earlier.”

Scientists are always looking for ways of spotting dementia earlier, as this is the key to better treatment.

The study involved 175 people, some of whom had neurodegenerative diseases.

They were shown videos of people talking, who were sometimes sarcastic or told lies.

People with frontotemporal dementia found it particularly difficult to spot the lies and sarcasm.

This could provide a useful early warning sign.

Dr Rankin said:

“If somebody has strange behavior and they stop understanding things like sarcasm and lies, they should see a specialist who can make sure this is not the start of one of these diseases.”

Other early warning symptoms of dementia include a change in sense of humour:

“Changes in sense of humour could be an early sign of dementia, a new study finds.

A shift to preferring slapstick humour — like Mr Bean — over satirical or absurdist comedy, such as Monty Python, could be an early sign of Alzheimer’s.

Friends and relatives of those with dementia reported seeing changes around nine years before the more typical memory problems.”

Another even more surprising sign of dementia is being unable to smell peanut butter.

The study was published in the journal Cortex (Shany-Ur et al., 2012).

Alzheimer’s photo from Shutterstock