Optimists live longer than pessimists. Considerably longer, according to research by the Boston University School of Medicine. And we're not just talking about a few extra days or weeks. In a study that involved 69,744 women and 1,429 men, researchers found that those with the highest levels of optimism had a life span that was 11 to 15pc longer than those with the lowest optimism scores. Those same high-scoring optimists also had a 50 to 70pc greater chance of achieving "exceptional longevity" - which is getting to 85 years of age.
"While research has identified many risk factors for disease and premature death, we know relatively less about positive psychological factors that can promote healthy ageing," said Lewina Lee, assistant professor of psychiatry at Boston University School of Medicine, when the results were published. "This study has strong public health relevance because it suggests that optimism is one such psychological asset that has the potential to extend the human lifespan."
Being optimistic means living your life in the belief that, in general, more good things will happen than bad. It's definitely a positive way to live, but can it really help you live longer?
Well, it seems that being optimistic can reduce the chances of getting many of the conditions that normally end lives early.
In a 2015 study, researchers at the University of Illinois examined the association between optimism and heart health in 5,100 adults aged between 45 and 84. Each participant's heart health was established by measuring their blood pressure, BMI, blood sugar and cholesterol levels, and by assessing their diet, level of physical activity and tobacco use. Each of these criteria were then given a score of 0, 1 or 2 - with higher scores indicating an ideal - which were then added together to give an overall heart health score.
Alongside this, each participant completed a survey to measure their mental health, including how optimistic they were.
The researchers found that those with the highest optimism levels were twice as likely of "being in ideal cardiovascular health" as those with the lowest levels of optimism. The optimists had better cholesterol and blood sugar levels, healthier BMIs, and were more likely to be physically active and less likely to smoke.
A few years earlier, in 2011, a study by the University of Michigan found that optimism can greatly reduce the risk of stroke. By devising a scale to measure the optimism levels of 6,044 adults over 50 years old, then monitoring their health over the next two years, the researchers discovered that every one-point increase in their optimism scale equated to a 9pc reduction in stroke risk.
Optimism also reduces the risk of diabetes. An American Menopause Society study that followed the health of 140,000 post-menopause women for 14 years found that women with high levels of optimism were 12pc less likely to develop type 2 diabetes.
But in addition to reducing the chances of having a serious illness, optimism also appears to reduce the severity of some illnesses and increases the chances of recovery.
In a small-scale study carried out by the University of Texas Health Science Centre at Houston, researchers found that, in a sample of 49 stroke survivors, those with the highest levels of optimism suffered less severe strokes, had lower inflammation levels - post-stroke inflammation can further damage the brain and hamper recovery - and were less physically disabled by the stroke than those with low optimism levels.
"Our results suggest that optimistic people have a better disease outcome, thus boosting morale may be an ideal way to improve mental health and recovery after a stroke," said Yun-Ju Lai, the study's lead author.
In fact, 30 years of research have shown that optimism can positively affect our health in a multitude of ways. Optimists have better immune function. They sleep better. They heal quicker. They enjoy greater IVF success. The list goes on.
But why is this?
While the exact mechanism behind optimism's effect on our health is not known, a number of important factors have been identified.
One of those is behaviour. In general, optimists tend to adopt behaviours that make it less likely that they will have a serious illness. Take exercise, for example. A US study of 73,485 women found that those with high levels of optimism were more likely to take part in vigorous physical activity, which helps reduce the risk of heart disease. And while those same women reported a drop-off in this activity in their 50s, they still had higher levels of vigorous physical activity compared to similarly aged women with low levels of optimism.
Optimists are also better at dealing with the stressors in their lives. While pessimists have a tendency to avoid dealing with their problems directly and employ harmful coping strategies - such as excessive drinking, optimists are more likely to make plans and confront their problems head on. And if they don't succeed, they'll try again.
Optimists are also less prone to rumination and worry, which is why they rarely suffer from insomnia, and they usually sleep for six to nine hours a night, according to research by the University of Illinois.
These differences in how optimists and pessimists deal with life's stressors lead to important changes in how their bodies react to those stressors. For example, studies have found that pessimists, who tend to have higher systolic and diastolic blood pressure anyway, "tend to have larger blood pressure responses to stressors." Optimists, however, have less severe physiological reactions to the same stressful situations.
Despite all the years of research, psychologists are still not in agreement about the nature of optimism. Some believe that it's a personality trait, which they call dispositional optimism. And that, bar some fluctuations, this personality trait remains relatively stable over an individual's life course. They know this because dispositional optimism is quite easily measured, often by using a simple 10-question survey called the Revised Life Orientation Test.
But there are some psychologists who believe that optimism is a thinking style. And one of the interesting things about thinking styles is that they can be learnt.
Does that mean that pessimists could improve their health by learning to think like optimists?
"You can definitely learn optimism," says positive psychologist Dr Jolanta Burke of Maynooth University. "We have a lot of evidence suggesting this. And once you have taught someone the skills, their optimism level will keep improving.
"It's just a different way of looking at things. We can all change the way we think about things. And the great thing is, by changing your thinking you also change your physiological reactions. You change your emotions. You change your actions."
A good way of illustrating the differences in optimistic and pessimistic thinking styles is to look at how optimists and pessimists react to failure in their lives.
"Pessimists tend to blame themselves for bad things," says Jolanta. "For example, if they don't do well on a driving test they'll say, 'Oh, I'm absolutely useless at this!'
"And when you say this, that you as a person are useless at something, the chances of even trying it again are significantly reduced."
Optimists, explains Jolanta, are more likely to recognise the different factors that contributed to the test failure - such as lack of practice - and then take the steps needed to get it right next time.
Optimists also believe that every failure, every bad situation, is temporary. "And this is really important," explains Dr Burke. "If you don't think it's temporary, you don't have hope for change. And we require hope to make things happen.
"Something we've seen consistently in our research is that people with optimism keep trying. They don't give in as quickly. Pessimists, on the other hand, when they're in a bad situation, they think it's going to be like this forever, getting them so down that they're less likely to re-engage with things."
Optimists are also able to compartmentalise the bad things in their lives. "Let's say an optimist failed a test. They'll say, 'Okay, it's just a test. I'm not a failure. It's just one small aspect of my life that did not go well. Look at all the good things I have. I have friends. I have a family. I have a good job.'
"They're able to look at things from a different perspective - a larger perspective."
The good news is that there are very few complete pessimists in the world. Most of us are pretty optimistic, even when faced with a serious illness. But sometimes, we need a little help in believing that good things will happen.
"The people we surround ourselves with have a huge influence on us," says Dr Burke. "In difficult times you need other people to get you out of your thinking. It's terribly important.
"Optimism needs to be viewed as taking control of your life. Pessimism creates a lot of inertia. It's better to be optimistic."