 With the explosive rise of empirical research on happiness, a central question is how far, and how, happiness might be  measured.[11]  There seems to be no in-principle barrier to the idea of measuring, at least roughly, how happy people are. Investigators may never enjoy the precision of the “hedonimeter” once envisaged by Edgeworth to show just how happy a person is (Edgeworth 1881). Indeed, such a device might be impossible even in principle, since happiness might involve multiple dimensions that either cannot be precisely quantified or summed together. If so, it could still be feasible to develop approximate measures of happiness, or at least its various dimensions. Similarly, depression may not admit of precise quantification in a single number, yet many useful if imprecise measures of depression exist. In the case of happiness, it is plausible that even current measures provide information about how anxious, cheerful, satisfied, etc. people are, and thus tell us something about their happiness. Even the simplest self-report measures used in the literature have been found to correlate well with many intuitively relevant variables, such as friends’ reports, smiling, physiological measures, health, longevity, and so forth (Pavot 2008).