Whatever metrics anyone comes up with today, cannot be scaled back historically. If e.g. you put some metric around "do you have a car", and try to use that me

Whatever metrics anyone comes up with today, cannot be scaled back historically. If e.g. you put some metric around "do you have a car", and try to use that metric towards "happiness", this implicitly puts all the people in 19th century in the "unhappy by not having a car" category. The only thing that can be used over time is the self-reported level of happiness. I.e. "on a scale of 1 to 5, how happy do you feel today?". And this is exactly what this study does. The real issue is that GDP (a concept which is too complicated to be understood by general populace), or any other "economic performance" criterion, have little to no relation to the self-perceived happiness of the population. Objectively poor people can be happy, and objectively rich people can be unhappy.