"I don't know how we do it, but we need to start rebuilding community as a social network of interpersonal support rather than merely a zip code."
The obvious, true, academically-established but politically-unacceptable answer is that we need homogeneity.
People give a crap about others in their tribe, who will have their back later, who are working towards the same social goals as them.
People don't give a crap about people of other tribes, who disrespect/hate them and their beliefs, who are working to move the world in an entirely different direction.
Robert Putnam at Harvard did a study around ethnic diversity and social trust/cohesion, hoping to find that diversity helped cohesion. But, he found the opposite - diversity eats away at public trust, far more perniciously than he thought or believed. He delayed publishing for years because this result appalled him so much. But, he's a good scientist, so he did eventually report his findings [1].
So, if you want that genuine sense of community togetherness, you need to:
1. Develop a distinct identity, with a clear definition for who is and who is not in the tribe. People being as they are, you need to develop a feeling of specialness in the in-group, and a sense of otherness towards outsiders.
2. Encourage group-signifying standards, practices, and rituals on members.
3. Do not admit new members who cannot or will not homogenize into those tribal signifiers.
4. Do not permit foreign allegiances among members.
Basically, do what Japan or Korea do now, or what the West used to do in the 50's, and what all pre-modern societies did.
II
Unfortunately it's hard to admit this, since feel-good wishful thinking about diversity is so pervasive. The idea that "diversity is our strength" is one of the Great Lies of the modern dominant cultural dogma. It's the exact opposite of the truth. Unity (while learning from others) is strength. Diversity is conflict.
And everyone knows this, when it's presented the right way. We all know that it was a great crime when the British and French deliberately drew Middle Eastern country borders to contain several tribes in one country. Iraq is in constant civil war because it was designed to be in constant civil war, by putting together Kurds and Shia and Sunni in one country. The British used diversity as a weapon on the Arabs, and it worked. It's still working. The damage never ends. Meanwhile, Iran is at peace, because it is reasonably homogenous. So is Pakistan, because they persecuted the Hindu population from 15% in 1948 to 0% today.
Diversity is not strength. In the Balkans, in Rwanda, in Chechnya and Syria and Iraq and pre-partition India/Pakisan and modern India and Aceh province and western China and Paris Banlieues and Molenbeek and Malmo and everywhere else, diversity is conflict.
So if you want unity, choose homogeneity.
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[1] a starting ref on Putnam's study: http://www.ft.com/cms/s/0/7e668728-5732-11db-9110-0000779e23...
Other reading: Coming Apart by Charles Murray