Objections to the paradox focus on the time series generalization, that trends in happiness and income are not related. In a 2008 article economists Betsey Stevenson and Justin Wolfers state that “the core of the Easterlin paradox lies in Easterlin’s failure to isolate statistically significant relationships between average levels of happiness and economic growth through time,” and present time series evidence of a significant positive statistical association between happiness and income.[6] A 2012 article by the same authors and Daniel Sacks returns to this time series criticism with new data, though at times the article asserts that the paradox is a contradiction between two types of cross section evidence -- data for persons and for countries.[7] Outside of economics, two founding fathers in the study of self-reported happiness, Ed Diener in psychology, and Ruut Veenhoven in sociology, have each, with their collaborators, also presented evidence of a significantly positive time series relationship.[8][9] A rebuttal by Easterlin points out that these studies do not focus on identifying long term trends; rather, they are based on time series that are short or have only two observations –in both cases insufficient observations to establish a trend. The positive association they present is that between the fluctuations in happiness and income, not the trends.[10]