SEATTLE — Have you ever looked on the bright side? Detected a silver lining? Are your glasses half-full or otherwise rose-colored?

Or do you take a dim view, fix on the dark clouds and brace for the worst?

There are optimists. And then there are the people who want to strangle them.

Which camp you fall in may have all kinds of consequences for your health — and that goes beyond your risk of getting throttled. Dozens of studies imply a bleak outlook somehow invites bleak outcomes, some as serious as a sluggish immune system, heart disease, even early death. New findings make matters worse by adding a pair of dreaded neurological diseases to the things a pessimist has to worry about.

Self-help books and folks who plain don't know what else to say when faced with a friend's misfortune have pushed the so-called power of positive thinking for decades.

"That's the pop psychology version of it, but there's a lot of hard science looking at how psychological processes affect our very biology," says Bonnie McGregor, a clinical psychologist and researcher at Seattle's Fred Hutchinson Cancer Research Center.

A growing area of research suggests the connection between attitude and health has more to do with the ravages of stress than the triumph of happy thoughts. That means learning to cope with anxiety may be more important — or at least more helpful — than trying to look on the bright side.

Not to be negative, but let's start with the bad news.

The most recent entry in the why-are-they-doing-this-to-me category of scientific research is the finding that pessimists are more likely to develop dementia than their blithe counterparts. A sister study found they also have a higher risk of Parkinson's disease.

"Psychologists love to blame pessimism for health problems, but I never really believed it until now," says Dr. Walter Rocca, a professor of neurology at the Mayo Clinic College of Medicine in Rochester, Minn., and a researcher on both studies.

Rocca and his colleagues followed thousands of Minnesotans who took a personality test in the 1960s. Those who scored highest on pessimism were about 30 percent more likely to show dementia up to four decades later. Another batch of Minnesotans revealed an even stronger link between pessimism and Parkinson's — a 50 percent higher chance of developing the degenerative disorder.

Both studies were presented at the annual meeting of the American Academy of Neurology. They don't show causality, Rocca stressed, just an association. In other words: "Pessimists, this doesn't mean you are doomed."

The studies weren't designed to answer the why question, but they still may provide a clue. The researchers also looked back at the Minnesotans' levels of anxiety on those personality tests and found a similar link between anxiety scores and the brain diseases.

It could be that a propensity for pessimism, anxiety and these diseases share a common risk factor — a gene or some quirk of brain chemistry.

Another possibility, Rocca says, is that a pessimistic outlook leads to more anxiety, which can disrupt levels of stress-related hormones or otherwise knock the body's endocrine or nervous systems out of whack. This much is known. Then, theoretically at least, this could trigger some cascade of events that damages the brain and ends in Parkinson's or dementia.

It's premature to start testing whether anti-anxiety drugs could prevent these diseases, Rocca says. But it's not hard to imagine why a pessimist might suffer more from stress than an optimist.

Psychologists define optimists as people who tend to expect the best; pessimists, meanwhile, assume the worst is yet to come. Some combination of childhood experience and genetics is thought to construct this frame through which we see most events.

It's a spectrum, with most people falling in the middle, though Americans tend slightly to the optimistic side. The United States was founded by idealists after all; pessimists likely took one look at the boat and said: That'll never make it.

That kind of attitude gives pessimists a double whammy of anxiety. First, they interpret events as more stressful. McGregor describes a scene where a friend is late for a dinner date. An optimist might think: Oh good, I've got a moment to catch my breath. But a pessimist might fret: Did she forget? Was she in an accident? Does she think my time isn't as valuable as hers?

Second, when faced with stress, pessimists often don't cope as well.

Optimists have a curious habit of seeing stresses as challenges and forging ahead. Pessimists obsess or give up, so daily stresses build up.

There's a huge body of evidence showing that chronic stress is rough on the body. It weakens the immune system and contributes to higher blood pressure, migraines, sleep deprivation, stomach problems, even skin breakouts.

When you think about it this way, it's no surprise that earlier research out of the Mayo Clinic found that pessimists are more likely to die prematurely.

Still, there are limits to the power of positivity.

In a study last year in the journal Cancer, Australian researchers followed 179 lung-cancer patients and found optimists didn't live any longer than pessimists.

McGregor argues curing cancer is a bit much to ask of optimism. "Most chemotherapy can't stop advanced lung cancer, why would they expect a psychological construct to have that effect?"

Telling people to think happy thoughts may not be a particularly useful way to fight cancer, McGregor says, but reducing the anxiety from negative thoughts might be.