"Optimists are more likely to live longer than those who have a more negative approach to life, a US study has found," BBC News reports. The Mail Online reports on the same study claiming that "Optimists are up to 70% more likely to live to be 85".

The study used information collected from male war veterans and female nurses taking part in 2 long-running studies in the US. The participants were around 60 to 70 years old when they completed optimism questionnaires, and the researchers looked at whether optimism was linked to living longer.

People who had the highest optimism scores had a lifespan about 9% longer than those with the lowest scores. But despite media reports, the most optimistic were in fact no more likely to live to age 85. The 70% figure came from a result that had not taken account of all influencing factors.

Ultimately this research cannot prove cause and effect. Both optimism and lifespan may be influenced by many hereditary, health, lifestyle and personal factors. People with a good overall standard of health and wellbeing are probably more likely to be optimistic about their future. While researchers attempted to adjust their analysis for these sort of factors, it's hard to remove their influence fully.

It's also questionable how applicable the results of these optimism assessments, taken 15 to 30 years ago from very specific groups of older US nurses and war veterans, are to the general UK population.

Nevertheless the research highlights the importance that mental health and wellbeing can have on physical health.

Where did the story come from?

The study was carried out by researchers from the National Center for Posttraumatic Stress Disorder of the Veterans Affairs Boston Healthcare System, Boston University School of Medicine, Harvard Medical School and other institutions in Boston, US.

It was funded by the National Institutes of Health, Clinical Science Research and Development Service, US Department of Veterans Affairs, and Fonds de Recherche en Santé–Quebec. It was published in the peer-reviewed journal, PNAS.

The UK media coverage was generally positive in tone taking the findings at face value without considering the many limitations of this research. Notably many of the media sources highlighted the claim that optimists are 70% more likely to live to 85, which is not correct.

Arguably the authors of the study are partly to blame for this, as they placed the headline-grabbing result, which did not adjust for all confounding factors, into the abstract.

What kind of research was this?

The researchers used data collected from 2 long-running US cohort studies. They looked at women participating in the Nurses' Health Study, and men participating in the Veterans Affairs Normative Aging Study, to see whether assessments of optimism were linked with lifespan.

Prospective cohorts can look at links between a risk factor or exposure and later health outcomes.

However, the data cannot prove direct cause and effect between the 2, particularly considering the 2 cohorts were not set up to examine this question. Many factors may influence both optimism and lifespan.