The research suggested that because older adults may have greater difficulty with cognitive flexibility, abstract reasoning and short-term memory, they also have greater difficulty with tests of humor comprehension.

Researchers tested about 40 healthy adults over age 65 and 40 undergraduate students with exercises in which they had to complete jokes and stories. Participants also had to choose the correct punch line for verbal jokes and select the funny ending to series of cartoon panels.

Findings were published earlier this month in the Journal of the International Neuropsychological Society.

The research conducted by graduate student Wingyun Mak and psychology professor Brian Carpenter showed that the younger adults did 6 percent better on the verbal jokes and 14 percent better on the comic portion than did older participants, Mak said.

But who decides what's funny?

The researchers, citing past work in the field, wrote that humor research is "rooted in the philosophical notion that humor arises from a sense of incongruity, a conflict between the expected and the actual."

"Successful comprehension of humor occurs upon resolving something that is seemingly incongruous with a logical but less obvious explanation."

Researchers used a verbal joke test developed in 1983 and used in other humor studies. Mak added a new element, though, by showing participants cartoons from the Ferd'nand comic strip, and asking them to choose between four panels to locate the funny ending. Three of the choices for each cartoon were the wrong ones, created by an artist for the study.

Participants had to respond to jokes like this one:

A businessman is riding the subway after a hard day at the office. A young man sits down next to him and says, "Call me a doctor ... call me a doctor."

The businessman asks, "What's the matter, are you sick?"

Participants then had to choose the right ending. For this one, the correct answer was "I just graduated from medical school."

Wrong choices were straightforward answers or conclusions that did not follow from the premise. Among the wrong answers: "Yes, I feel a little weak. Please help me."

"This wasn't a study about what people find funny. It was a study about whether they get what's supposed to be funny," Carpenter said.

"There are basic cognitive mechanisms to understanding what's going on in a joke. Older adults, because they may have deficits in some of those cognitive areas, may have a harder time understanding what a joke is about."

Mak said humor comprehension merits further study because of the potential physical and psychological benefits of humor.

"I think it's really important to note this doesn't mean older adults aren't funny or don't understand humor," Mak, who is from Los Angeles, said.

She said humor comprehension and humor appreciation are tested in different ways. The Washington University study didn't delve into humor appreciation. In fact, Mak said, older study participants who may have picked the wrong answers may also have been laughing at their choices at the time.

Josephine Bertani, 73, of St. Louis, took part in the study. She has some memory issues, and thinks she probably didn't come up with all the right answers.

Even so, she leads tours of senior citizens as a volunteer through St. Ambrose Catholic Church.

"I'll entertain them with jokes," she said, adding that some tour participants bring her jokes that she can read aloud on bus trips. Any cases of the older people not laughing? "Not yet," she said.