Americans are spending less time outdoors:

Specifically in 2012, Americans spent one day less traveling and one week less in buildings other than their homes when compared to a decade earlier. The trend of staying indoors is especially strong for those ages 18 to 24: the youths spent 70 percent more time at home than the general population. At the other end of the age spectrum, those 65 and older were the only group that spent more time outside the home compared to 2003.

Numerous factors could be influencing this. For example, people might be glued to their smartphones. But that alone does not explain it; a more significant factor is what Robert Putnam noticed about diversity: it causes people to withdraw from society at large and stay isolated, atomized, and withdrawn because there is a lack of social trust:

Harvard political scientist Robert Putnam — famous for “Bowling Alone,” his 2000 book on declining civic engagement — has found that the greater the diversity in a community, the fewer people vote and the less they volunteer, the less they give to charity and work on community projects. In the most diverse communities, neighbors trust one another about half as much as they do in the most homogenous settings. The study, the largest ever on civic engagement in America, found that virtually all measures of civic health are lower in more diverse settings. …Putnam claims the US has experienced a pronounced decline in “social capital,” a term he helped popularize. Social capital refers to the social networks — whether friendships or religious congregations or neighborhood associations — that he says are key indicators of civic well-being. When social capital is high, says Putnam, communities are better places to live. Neighborhoods are safer; people are healthier; and more citizens vote. …Higher diversity meant lower social capital. In his findings, Putnam writes that those in more diverse communities tend to “distrust their neighbors, regardless of the color of their skin, to withdraw even from close friends, to expect the worst from their community and its leaders, to volunteer less, give less to charity and work on community projects less often, to register to vote less, to agitate for social reform more but have less faith that they can actually make a difference, and to huddle unhappily in front of the television.” “People living in ethnically diverse settings appear to ‘hunker down’ — that is, to pull in like a turtle,” Putnam writes.

This “hunkering down” can explain why people are staying home instead of going to the movies, watching sports instead of participating, and otherwise withdrawing from society. Without diversity, there are clear standards of behavior and so people can be confident in their actions, knowing what will produce good and bad responses.

With diversity, there are many conflicting values systems and groups participating in a cold civil war for dominance, which means that being out in the world is a threat because at any time, another group may attack without warning or one may be publicly shamed within one’s own group for not knowing the myriad values and customs of the combined populations.