> study after study shows that humans very much define happiness in relative terms

Whether or not the gap is widening, that definition of happiness is very problematic. There's something wrong when, as an example, someone is happy with a "rice-and-beans" lifestyle when all their neighbors have the same lifestyle but is unhappy with a better "chicken-in-every-pot" lifestyle because some of their neighbors have caviar and foie gras (insert your preferred luxury foods).