New research suggests that shifts in what a person finds funny can herald imminent changes in the brain—possibly presaging certain types of dementia

A close friend was an even-keeled, responsible man, endowed with a sunny outlook and a gentle, punny sense of humor. So when he started to make snide remarks at social gatherings several years ago, I secretly championed the delight he was taking in his newfound freedom from social constraints. After more than 50 years of exemplary adult behavior, he had earned the right to play court jester now and then. Or so I thought.

New research from University College London suggests that shifts in what a person finds funny can herald imminent changes in the brain. Published this month in the Journal of Alzheimer’s disease, the study found that an altered sense of humor can predate a diagnosis of dementia by as much as 10 years. A burgeoning penchant for slapstick—over a past preference for satire or absurdist humor, for example—characterized nearly everyone who eventually developed frontotemporal dementia. (Far less common than Alzheimer’s, this illness usually hits people in their 50s and 60s.) But a changed sense of comedy affected less than half the people later diagnosed with Alzheimer’s disease.

“The type of change could be a signpost of the type of dementia the person is going to develop,” said Jason Warren, a neurologist at University College London who led the study. Acknowledging that humor is an unconventional way to think about neurodegenerative disease, he told me that most research in the area uses more standard assessment tools, such as memory tests, but that “memory may not be the type of thing that patients or relatives notice first.” Such warnings could be subtle changes in behavior, including humor.

After all, most forms of humor require some form of cognitive sleight-of-hand. “Getting” satire hinges on the ability to shift perspective in a nanosecond. Absurdist jokes play fast and loose with our grasp of logic and social norms; black humor lampoons taboos. All are a rich source of data about the brain.

“Humor is like a stress test,” said Dr. Warren. “The same way you’re on a treadmill to test the cardiovascular system, complex jokes are stressing the brain more than usual.”

Modest in size, the London study compared 48 patients from an outpatient dementia clinic with 21 healthy older adults. A spouse or longtime caregiver filled out a semi-structured questionnaire about what kinds of TV shows, comic writing and other media each subject preferred. Fifteen years before the study and now, how much did the person enjoy slapstick (along the lines of “The Three Stooges,” though the British study focused on U.K. entertainment only, like “Mr. Bean”), satirical comedy (“Saturday Night Live”) or absurdist comedy (“Monty Python”)?

A change in the type of comedy that people found funny turned out to be a sensitive predictor of a later diagnosis of frontotemporal dementia, though Dr. Warren cautioned that a small, retrospective study like this one is just a first step. Still to come are brain-imaging studies and a prospective look at changes in humor in people who carry genetic markers for the disease.

Their findings could well apply to me, given that dementia runs in my family. I admit, though, that I’m not used to thinking about humor this way. The quip attributed to Groucho Marx that “a clown is like an aspirin, only he works twice as fast” captures my view.

In a perfect world, laughter would be the antidote to illness, not its red flag.