If you're not laughing, it may be an early sign of dementia.
HEALTH It seems some people are funny by nature. They find irony in nearly every social setting. Others in the same situations fail to see the humor and may, in fact, chastise the drollery one as buffoonish. Is the spontaneity of humor attributed to genetics or environment?
Types of Humor
- Affiliative, use of humor to amuse others and facilitate relationships
- Self enhancing, use of humor to cope with stress and maintain a humorous outlook during times of difficulty
- Aggressive, use of sarcastic, manipulative, put-down, or disparaging humor
- Self-defeating, use of humor for excessive self-disparagement, ingratiation, or defensive denial
The Science of Humor
Why is it that when two people view the same comic strip, one can find it hysterical while the other considers it stodgy? Scientists have been performing serious research to uncover the answer. Many professions extol the virtue of humor beyond recreational entertainment, into the areas of education, public speaking, and diagnostic medicine.
An article published in New Scientist concluded that humor is not genetic after researchers studied reactions of 127 pairs of female twins. Seventy-one pairs of twins were identical with matching genes vs 56 fraternal, sharing about 50 percent of their genes. The Twin Research Unit, St. Thomas Hospital, London tested reactions to cognitive cartoon slides by asking participants to rate them on a scale of zero to 10. The research revealed that identical twins shared no more common responses to the jokes than did the fraternal twins. 
Health Benefits of Humor
Even religion has sought to address the perplexing phenomenon of humor as these excerpts from Awake! magazine reveal. 
"Some believe that at the heart of humor is incongruity — the bringing of two seemingly incompatible elements together. A grown man dressed as a circus clown may throw a small child into a fit of laughter. However, an adult with his greater life experience and superior cognitive skills may no longer find the clownish antics funny. He may find pleasure in more mental forms of humor — puns, plays on words, or jokes — that exploit incongruity on a verbal, rather than a physical, level.
"Some researchers believe that humor may result from the release of pent-up emotional energy. Humor may serve to mask tension and pain. Says the Bible: "Even in laughter the heart may be in pain; and grief is what rejoicing ends up in." (Proverbs 14:13) Evidence is slowly accumulating that laughter may even serve as a therapeutic tool...."Negative emotions can manipulate the immune system, and it now seems positive ones can do something similar." This underscores the wisdom of the Bible's words: "A heart that is joyful does good as a curer." —Proverbs 17:22 NWT.
Humor in Diagnostic Medicine
Frontotemporal dementia (FTD) is the second most common form of dementia in people under 65. Researchers from the University of New South Wales (UNSW) say patients with FTD or Pick's disease (one type of FTD ), have trouble reading emotions and are often unable to sense when someone is being sarcastic. Patients are often misdiagnosed with a personality disorder or dismissed as strange and ostracized for rudeness or lack of empathy. 
In the UNSW study, actors perform short scenarios, always using exactly the same words — in some of the scenarios they are sincere and genuine and in other scenarios they are being sarcastic. Then patents were simply asked: Do you get the joke?
Senior author, Professor Hodges, says the test showed that people with FTD are very literal because the differences were all about tone of voice, mannerisms and subtle social cues. When the test was tried on Alzheimer's patients, they had no trouble with it and recognized the sarcasm. The researchers showed these changes in emotion correlated to brain shrinkage in three closely integrated brain regions and Professor Hodges says all the findings were consistent with shrinkage of the amygdala — an area on the right side of the brain which controls a person's mental and emotional state. 
What is Sarcasm?
One dictionary defines sarcasm as the use of irony to mock or convey contempt intended to offend someone. This differs from sardonic, which is grim mocking or a cynical derision either verbally or facially without necessarily any intent to offend or cause emotional distress. Irony comes from an amusingly provocative disparity between any two seemingly incompatible things, expressed to amuse rather than to offend; note the irony in the following examples. 
- There are three type of people in the world: Those that can count and those that can't.
- I have reservations that the restaurant will accept reservations.
For some, a clearer distinction is made between sarcasm and sardonic by considering the former is an insult intended to illicit verbal response or physical reaction, where the latter precipitates contemplation or changed mindset. As defined, these statement could be considered sardonic:
- If peace were measured by the number of dead, we should get the Nobel peace prize.
- Consumer Reports torpedoed sales of iPhone 4 when they failed to recommend it.
Below are some examples of sarcasm to informally test for early signs of dementia. 
- Don't be humble. You're not that great.
- He has no enemies, but is intensely disliked by his friends.
- I like you. People say I've got no taste, but I like you.
- Sometimes I need what only you can provide: your absence.
Some people can't laugh to save their life. How did you do? Are you amused by any of the sarcastic statements? In some cases the ability to see the humor in sarcasm is predicated by the target of the insult. In other cases, one might detect the sarcasm but fail to see the humor. For a more formal Humor Style test, visit the Psychology Today website. 
The debate about whether humor is genetically or environmentally disposed is not over. In every school there's a class clown. In families of siblings there is often the "odd" one. Whether you like humor or not, it is here to stay, as long as you can remember to laugh.
- How Do You Use Humor? psychologytoday.com
- What Humor Style Do You Have? shockmd.com
- Nurture, Not Nature. Environment, Not Genetics Defines Humor. facstaff.uww.edu
- Put Humor Into Your Life. Awake! Watchtower Bible and Tract Society, May 22, 1994
- Alzheimer's Facts and Figures 2010. Page 6. alz.org
- Sarcasm — a diagnostic tool for dementia. news-medical.net
- Sardonic or Sarcastic? (And What About Ironic?) alphadictionary.com
- Sarcastic Quotes and Sayings. coolnsmart.com
- Globalization: The Sociology of Humor and Laughter. wais.standford.edu
- Blondes and Airplanes. jokes.com
- Jokes. onsetsoftware.com