To assess causality the authors looked at 30 studies which measured happiness and which undertook long term follow ups. They also chose only to include studies which controlled for health - as health has such a large impact on longevity. The result were that being happy had a similar effect on life span as not smoking.
The authors suggest that the findings have implications for public policy: for policies which promote happiness. While the evidence does suggest that happiness has many beneficial effects, how we go about increasing happiness is another matter. Happiness is best worked for indirectly, it is a mood which is contingent on other things and should not be boosted directly. This means that any policy which places happiness at the heart must take this into account. To read the article click here