[keyraycheck]

or is it other way around? i.e. High performers are happier and more optimistic. So cause and affect is reversed

[jimkleiber]

I'm not sure if your comment was in response to my comment, as I was trying to say that I believe happiness != well being != optimism.

[endymi0n]

The study answers this question (at least partly) by measuring happiness levels _before_ the job.