The graph depicts the results of a pilot study comparing "being right" vs. "being happy," as reported in the Christmas issue of the British Medical Journal:
This might be the first study to systematically assess whether it is better to be right than happy; a Medline search in May 2013 found no similar articles. Our null hypothesis was that it is better to be right than happy.
To be eligible participants had to be part of a couple and willing to take part in the study... It was decided without consultation that the female participant would prefer to be right and the male, being somewhat passive, would prefer to be happy... The female participant was blind to the hypothesis being tested, other than being asked to record her quality of life.
The intervention was for the male to agree with his wife’s every opinion and request without complaint...
Two participants were eligible and both (100%) were randomised... The data safety monitoring committee stopped the study because of severe adverse outcomes after 12 days. By then the male participant found the female participant to be increasingly critical of everything he did. The situation had become intolerable by day 12...
The results of this trial show that the availability of unbridled power adversely affects the quality of life of those on the receiving end...The study has some limitations. There was no trial registration, no ethics committee approval, no informed consent, no proper randomisation, no validated test instrument, and questionable statistical assessment. We used the eyeball technique for single patient trials which, as Sackett says, “more closely matches the way we think as clinicians.
More details of the methodology and discussion of the results at the source. For those unfamiliar with the BMJ, it's worth pointing out that this venerable medical journal has a long tradition of publishing papers in the Christmas issue that are written somewhat che(tongue)ek. See for example my 2010 posts on Mozart's 140 causes of death and Crocodile forceps? or alligator forceps?