Scientists have only recently started carefully testing Thayer's century old idea in controlled experiments to see if it's really as universal as he claimed.
A team of researchers from the University of New York conducted a randomized control trial with 691 depressed patients from 83 physician practices across England.
We have the first early results on productivity from these tools and one is a controlled experiment that was done with people writing business writing.
One drawback with the studies is that they are all retrospective, looking back at naturally occurring data rather than randomised-controlled experiments.
Peter Rothwell at the John Radcliffe Hospital in Oxford and his colleagues looked at deaths due to cancers during and after randomised trials of daily aspirin.