Obviously a lot of things go into a long life but the authors feel like they are early in positing that positive psychosocial factors promote healthy aging.
Like all epidemiology, this is only correlation and simply exploratory. Don't begin to worry if you are not as happy as people on Facebook pretend to be you are at greater risk of early death. Their data used self-reported happiness from 69,744 women and 1,429 men who filled out surveys to state their level of optimism, as well as their self-reflection about their overall health and health habits such as diet, smoking and alcohol use.
The women filled out the diaries for 10 years and the men for 30 years.
By statistically comparing initial levels of optimism, the researchers found that the most optimistic people demonstrated, on average, an 11 to 15 percent longer lifespan, and had 50-70 percent greater odds of reaching 85 years old compared to the least optimistic groups.
If you believe that optimism is why people live longer, you may also be interested in buying expensive yogurt, supplements, and organic food.
There are obviously numerous confounders in that. You may be less optimistic for lots of reasons and you may have turned out to be right. Like BMI and other population metrics that are laid over individual behavior, it doesn't have much value in a bubble, especially because this was persistent despite accounting for age, demographic factors such as educational attainment, chronic diseases, depression and also health behaviors, such as alcohol use, exercise, diet and primary care visits.
There is no plausible biological hypothesis
How would optimism create longer life? The advantage of exploratory statistical papers is the lack of need for science.