According to a new study, people with a certain type of dementia are limited in their ability to recognize lies and sarcasm. In fact, this inability can be observed notably before any of the usual symptoms of neurodegenerative conditions appear, meaning it can be used not only to diagnose dementia, but to predict it.
Failing to detect insincerity is more than just a social liability. People who can’t tell when they’re being lied to are easy marks for scam artists and confidence tricksters, who often prey on the elderly and vulnerable.
The frontal lobe is the part of the brain that detects insincere communication. In a condition called frontotemporal dementia, that part of the brain—along with the temporal lobe—undergoes a gradual decline. This type of dementia strikes only about five percent of sufferers, and often begins between the ages of 40 and 70.
The associated decline eventually leads to impulsive and socially unacceptable behaviors. Before that, however, scientists now say it leads to gullibility and an inability to tell when people are not being sincere, whether than means deliberate deception or sarcasm.
“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,” said Katherine Rankin in a statement. Dr. Rankin, a neuropsychologist at the University of California, San Francisco, was the lead researcher on the study.
There is no surefire way to predict which patients will develop neurodegenerative conditions, including dementia, but with this study, doctors may have a much higher success rate. The researchers say this inability to detect insincerity is a reliable sign of degeneration, though by no means a universal one, and it can help identify which patients would most benefit from close monitoring.