A sunny disposition isn't just good for your mental health. It's good for your body, too. It can even add years to your life. Sarah Pressman , a health psychologist at the University of California, Irvine, has spent her career investigating the link between positive emotions and physical health.

In the 2019 Annual Review of Psychology , she and her colleagues explore why a positive outlook generates physical health benefits . Knowable asked her about some of the high points, and how doctors and their patients can make use of the knowledge. This conversation has been edited for length and clarity.

How did you get interested in studying this?

For decades, researchers have been studying all the detrimental ways that stress can make us sick and lead to pain, and minor and major illness. As a graduate student, I got interested in the opposite: What can protect our bodies against the harmful effects of stress? At that time, in the early 2000s, the field of positive psychology was really just starting. I saw a natural synergy there — there are these positive factors, and maybe they could be protective against stress and have health benefits, or at least protect us against health harm.

And does a positive outlook make a measurable difference?

The negative effect on your health of being socially isolated is stronger than the effect of being overweight, a regular smoker or a heavy drinker. That kind of comparison hasn't been done yet in positive emotion research. But there's a host of studies — probably in the dozens now — that show that people who are more positive tend to live usually five to 10 years longer than those individuals who are less positive. That's a pretty large effect.

What causes this effect?

We have a lot of hypotheses. Positive emotion changes our stress perception so stressors don't seem as bad. It changes how we react to stressors, and it helps us recover. Both our stress reaction and our stress recovery have been shown to predict important outcomes. Pick a disease — heart disease, for example. If you feel calmer, your blood pressure is lower, your heart rate is lower. And we know one of the things that predicts heart disease is arteries blocked up with plaques. And where do those plaques come from? Partially, from damage from high-speed, high-pressure blood. If your blood pressure is lower, and your heart rate is lower, you have less of that turbulent blood flow, and therefore over time you might have less damage to arteries and less plaque.

Positive emotions also change how our immune system works. We don't know exactly how, but we do know that if I make you feel positive, if I make you feel calm, we change the numbers of your immune cells, and we tend to drop your inflammation level. For example, there's a marker of inflammation called interleukin 6, or IL-6. People who are generally more positive, or who are induced to feel more positive, have lower levels of IL-6 .

But even aside from that, when we are feeling positive, we're much more likely to engage in healthier behavior. We take better care of ourselves, we're more likely to sleep better and exercise, we have a better diet. People who are more positive tend to have more relationships, better-quality relationships. They're more likely to be married and stay married for longer. If you have good relationships, those people will encourage you to take care of yourself.

That gives us some really compelling pathways for how this can happen, both on the behavioral end and by directly altering cardiovascular function, hormonal function, immune function. If I'm happy today, that doesn't mean I'm going to live longer. But if I'm happy for a few years, that might make a difference.

How do we know that positive emotion causes better health, rather than the other way around?

To do the perfect study would require that we experimentally assign people to an intervention that makes them happier, or less happy, and see if that affects longevity. That has not been done. But we have a lot of studies of groups of people where we know the health and the emotional state of each person at the start. We control for sociodemographic factors, we control for medications and immune function. So we know that those people who were less happy at the beginning weren't less happy because they were already more sick .

Then we can look over time. If you control for smoking and health at the start and you still see the effect of positive emotion five or 10 years later, it's more suggestive than a study looking at people at just one point in time and just saying, “Oh, happy people feel healthy.”

Have you also done experiments?

We measured people's naturally occurring positive emotions. Then they were experimentally wounded. It was kind of a nasty study, actually. We damaged their skin by putting tape on it over and over and ripping the tape off. We monitored to see how quickly water was being lost from the skin surface. As that water loss decreases, we know the skin cells are healing. This is really an immune-system function test, because the more quickly your immune system is able to traffic white blood cells to the injury, the faster you will heal. We saw about a 20 percent shorter healing time for those individuals who were more positive versus those who were less positive.

There is another study, not yet published, where we manipulated positive emotion. There's something called the facial feedback hypothesis, where if you fake an emotion, it sends a message to your brain that you're feeling that emotion. If we trick people into smiling by holding things in their mouth, it can trigger a positive emotion.

So we had people smile while getting a fake flu shot. Some people were smiling and others were not. Those who were smiling had about 40 percent less pain from that needle, and their heart rate recovered faster from the stress of it.

Do we know that positive emotions — and not just the absence of negative ones — are causing the benefit?

That we actually know really, really well. Through the last 20 years of research, almost every study does a good job of accounting for that by controlling for negative emotions.

Time and time again, you see that it really does seem to be the presence of positivity , independent of negativity, that's driving health effects. It's the presence of positive emotions, not the absence of negative ones, that can help undo stress. If I have to give a talk and I'm feeling neutral, that isn't helping me — but if I can say, “Actually, I'm really excited about giving this talk,” that can change my stress trajectory . That's very different than the absence of a negative emotion.

Are there health conditions where a positive attitude doesn't help?

For individuals who have a serious chronic illness that's far gone — stage 4 cancer, end-stage kidney disease — the data are inconsistent. Some studies show benefit, some show harm, some show no effect. If we're talking about a minute immunological change from laughing, that's not going to kill millions of cancer cells.

On the other hand, if you are feeling hopeful and positive, and able to adhere to your doctor's recommendations, and take the medications that you're supposed to, and exercise when you're supposed to, and quit smoking, those things are helped by positive emotions, and can have an important role in helping at earlier stages.

This is something we have to work on, because if people want to positive interventions for these severe diseases, we have to really understand when it will be helpful. That's a really important next step for the field.

Isn't there a risk that people with serious diseases will be stigmatized into thinking it's their own fault for not being more positive?

We certainly don't want to say that. There's absolutely no evidence in health psychology that being unhappy causes cancer, or causes disease to happen. If someone gets diagnosed with cancer, you don't want to tell them to be happy all the time. There's good evidence that keeping negative feelings locked up inside is harmful to our health. They have to go somewhere. You have to let it out — express your negativity and process it. Once you've accomplished that, we can try to teach you how to find benefit.

It is very important for people to deeply understand the power of mind over body, because if you are depressed and you are stressed it can be hurting you, and we want to help you cope with that. There is value in pursuing happiness. It's not a selfish, silly, soft thing that you don't have to do. This is actually an important piece of being a healthy human. And at a time when your health is compromised it can be especially important.

Are there ways to change people's happiness level? Aren't some people innately Eeyores and others Poohs?

Some work suggests that as much as 40 percent to 50 percent of happiness is based on genetics — you just luck into being born a more positive person. But that leaves a lot of room to manipulate.

A good amount of our day-to-day wellbeing — maybe 30 percent to 40 percent — is due to how we choose to spend our time. We can choose to spend our time on things we know improve positive emotion, like spending time with the people we love, having good relationships, getting enough sleep, exercising.

But on top of that, there are some specific, well-researched interventions — little tweaks that can help you focus on positive things. We can train our brains to hang onto positive emotions, which should help promote that positive emotion in our daily lives. Some of the more popular activities are gratitude exercises, where before you go to bed you write down three things you're grateful for, and meditation.

The nice thing about happiness is you don't have to buy some expensive medicine. Much of this is free. Happiness is not just a luxury that rich people should be pursuing — it's something that absolutely everyone should be investing time in every day.