Heterogeneity of appearance erodes a group's moral growth
When a society is heterogeneous to the naked eye, it winds up stunting their moral sense, or corrupting it if it was already developed. There is simply too strong of a temptation to equate all that's going wrong in the world with the people who don't look like us. They clearly have some kind of inner, essential difference -- and maybe that's also causing them to be so screwed up. It's the same way we distinguish a poisonous from an edible species of plant.
We see this most depressingly in racially diverse countries, where people of one group complain about the other groups, whether justifiably or not. That removes the impetus to examine your own group and your own self, which is the basis for the concept of sin and redemption. For those concepts to take root, it needs to be an ethnically homogeneous group that nevertheless shows a range of behavior across individuals, from helpful to harmful.
As far as I can tell, Zoroastrianism was the first religion to develop the themes of sin and redemption, and not surprisingly that was when the Iranian pastoralists saw that some of their own kind were choosing to make a living by exploiting others, by raiding on horseback, rather than devoting their energies to tending their livestock. To their eyes, the criminals had the same inner essence as the stewards, and so both would have seen the pay-off of a life of crime. However, since some did and some did not follow that path, it must have been an act of free will on both of their parts. And likewise, they could return to the proper path by freely choosing to leave behind their exploitative ways.
Christianity perfected these concepts, also during a time when the cause of Jewish suffering was not so much those from outside, but the mutually antagonistic factions within the Jewish population. Rome did not turn Jewish women into prostitutes, and it was a sub-species of Jews -- not outside occupiers -- who had welcomed the money-changers into the Temple. Seeing that they were capable of bringing ruin upon themselves, some Jews lent their ear to Jesus. He did not obsess over repelling and expelling a foreign evil, but persuading every individual to check their internal temptation toward sin, to prepare for the coming end of days, when their repentance would save them.
At the other extreme, the Han Chinese have never discovered the concepts of sin and redemption as described above. Most of their moral systems are based on filial piety or at most an elaborate code of etiquette to maintain harmony. The closest they got was Taoism, which although very cryptic still cannot conceal its preoccupation with an individual swerving off of the right path and acting to get back on track. Not coincidentally, this philosophy or religion was developed during the Warring States period, when the Chinese had plenty of evidence right under their noses that evil didn't just come from some wicked band of foreigners.
Throughout their history, though, it was outside tribes who looked and acted very differently that brought misery to the Chinese -- namely the numerous pastoralist Steppe groups, whether Hun, Mongol, or whatever. This unrelenting pressure from raiding foreigners has fed the primitive equation of evil with foreign substances. Aside from the civil strife that gave birth to Taoism, they have not experienced enough internal breakdown to over-ride man's instincts toward chauvinism.
We can hardly ignore the parallels in the present day. Anyone who does openly think or talk about crime and depravity in America tends to focus too much on the disproportionate number of blacks and Hispanics among criminals. Well that's true enough, and it is worth reminding people of, given how politically incorrect it is to mention. But it's not as though Wyoming and Vermont were spared from the crime wave of the '60s through the '80s. Ditto the European countries with virtually all-white populations -- that wave of violence was international. And obviously the previous two international waves of violence had nothing to do with non-European groups, taking place from ca. 1580 to 1630 and then again from ca. 1780 to 1830.
Indeed, the best academic work on the heritability of crime comes from lily-white Scandinavia. They know that there weren't enough dark-skinned people there to have launched and perpetuated the most recent crime wave. It must have been some of their own. Their behavior genetics research has uncovered differences across individuals, not groups, in criminal tendencies.
As America grows ever more diverse, I fear we're heading in the direction of the Chinese, where eventually whites will, however secretly, think of and put the blame on blacks, Mexicans, etc., for what's going wrong with the country, regressing away from the focus on sin and redemption. In the good old days, not many white Americans lived near large concentrations of more criminally prone racial groups. Pretty soon we'll feel like the Mongols are storming in from the Steppe, and adopt the pack moral mentality.
To end with an example not at the level of race or ethnicity, just look at what's happened between economic classes, with the erosion of fellow-feeling under a tide of antipathy. That's happened in all civilizations before, but this time it's even nastier because class differences are visible to the naked eye -- namely how fat or lean people are. Obesity is caused by carb-scarfing, widely promoted by the know-nothing health experts, but some people are hit worse than others.
In particular, lower-class people have more addictive tendencies, while upper-class people are better able to defer gratification. So when the experts encourage us all to chow down on an addictive substance like carbohydrates, the lower-class people will fill up their shopping carts with cookies, chips, ice cream, pizza, soda, beer, etc. Upper-class people today are more likely than their counterparts 30 years ago to buy granola, fruit juice, rice pilaf, and so on, but most of them (except for the vegans) have an aversion to loading up on only sugar and starch. They don't have as much of a sweet tooth. They'll work in non-starchy vegetables, cheese, fish, chicken, sometimes even red meat -- but at any rate, real food, not just junk food.
So, just as alcoholism and the crack epidemic have been mostly lower-class afflictions, so has the obesity epidemic. Now it's not so hard for upper and lower-class people to tell who's who, even from far away. This stark difference in average appearance once more leads us to equate degeneracy with some out-group, like poor fat hicks. As with the Chinese viewing the Mongols, or whites viewing blacks, it's not that this equation doesn't have a strong basis. Lower-class people do have poorer impulse control and are fatter.
But now we've shifted focus away from our own addictions, like being plugged into one glowing box or another all day long for no good reason, just to feed our brain a steady flow of informational morphine. I mean, hey, no need to turn our lives around even a little bit -- it's not like we're poor fat hicks or anything.
This obesity-based hostility has become most visible at Disneyland and Disney World, two places formerly associated with bringing Americans from all walks of life together. Unlike most other public spaces, the classes still do mix there, but not very amicably.
I haven't been since the late '80s or early '90s, but my two brothers took my nephew two summers ago. By far the most remarked about thing was how many fat people in scooters there were clogging the space of pedestrians. Lots of others on the internet have noticed this too; one even said that he couldn't go on the "It's a Small World" ride because fat people broke it. By now they'll need to rebuild it as "It's an Extra-Extra-Large World".
Some sources of class divisions we just won't be able to do much about, like those that are part of very long-term historical trajectories. But cleaning up the place of addictive substances is worthwhile. They devastate the lower classes so much more than the upper classes, that it exacerbates the other class divisions, and makes it easier to tell who's who at a distance, luring us into equating deviance with outsiderness. I know the top-down approaches haven't worked much in the past, so maybe a grassroots appeal to leave behind their self-destructive ways is more in order. Whatever it is, though, it would go a long way toward minimizing the alarming between-group hatred.