It is extremely problematic to claim that cooperation declined in America after the 1960s without even mentioning civil rights. Americans are certainly cooperating more insofar that it’s no longer literally illegal for gay people to have a relationship, or that black families on vacation no longer need a book indicating which gas stations and hotels will actually serve them.
The fact that white people are meaner to each other - particularly liberal v. conservative - than they used to be is in fact directly (albeit not solely) related to this, as with the polarization in formal political parties. Seeing the decline after the 1960s is really not a mystery if you think about how American society drastically changed due to Democrat-led legislation passed in 1964 and 1965, right when the graphs started nosediving.
The fact that white people are meaner to each other - particularly liberal v. conservative - than they used to be is in fact directly (albeit not solely) related to this, as with the polarization in formal political parties. Seeing the decline after the 1960s is really not a mystery if you think about how American society drastically changed due to Democrat-led legislation passed in 1964 and 1965, right when the graphs started nosediving.