First they examined how life satisfaction was related to how much money each person earned. They found however that satisfaction was much more strongly related to the ranked position of the person's income (compared to people of the same gender, age, level of education, or from the same geographical area).
The results explain why making everybody in society richer will not necessarily increase overall happiness -- because it is only having a higher income than other people that matters.
The three authors of the paper were Chris Boyce, Gordon Brown (both of the University of Warwick's Department of Psychology), and Simon Moore of Cardiff University.