Many different studies have examined happiness in various countries. These studies generally rely on extensive surveys of inhabitants and their stated level of happiness. One, the Happy Planet Index, considers happiness in tandem with a country's environmental impact and carbon footprint.
The Day Reconstruction Method asks subjects to rank activities they performed on an "enjoyment scale" [source: Stehr]. The survey aims to be more accurate than previous ones by soliciting opinions about people's daily lives immediately following the events in question, rather than taking a holistic, long-term view of their overall contentment. The U.N. Human Development Index uses a mixture of data about gross domestic product (GDP), education and health to produce a measure of a country's success.
Unfortunately, broad studies on societal happiness are notoriously fraught with problems. From the phrasing of questions to the weather on the day that a survey is performed (respondents are generally more upbeat on days that are literally sunnier), numerous variables can be difficult to account for.
Even within a relatively happy country, like the United States, you can have a high level of "happiness inequality" among different social groups. However, a study published in July 2008 showed that, at least in the United States, the happiness inequality gap among people surveyed had decreased over the last few decades but that happiness inequality was increasing based on education level. More educated subjects reported higher levels of happiness than those who were less educated, which coincided with a growing income disparity in the country. But the happiness inequality between men and women and among different races had also lessened, the study found [source: Wolfers].