We go back to Arthur C. Brooks' 2007 What Really Buys Happiness? Not income equality, but mobility and opportunity
Most of the studies he discusses avoid defining happiness, and just let people report. At issue, of course, is that as long as one is not in dire straits, happiness and contentment in life depend on non-economic factors: temperament and personality, relationships, family, hobbies and interests, etc.
There is no one "happiness." Some are most happy when taking risks, others more comfortable when they feel more secure. Material gain is not a universal goal.