They use what they call "the savanna theory of happiness" to explain two main findings from an analysis of a large national survey (15,000 respondents) of adults aged 18 to 28.

First, they find that people who live in more densely populated areas tend to report less satisfaction with their life overall. "The higher the population density of the immediate environment, the less happy" the survey respondents said they were. Second, they find that the more social interactions with close friends a person has, the greater their self-reported happiness.