The siren song of homogeneity is a powerful one. On Twitter and elsewhere, I am encountering more and more young people (mostly men) who openly yearn for a society where everyone is white. The more reasonable among these young people tell me that homogeneity reduces conflict, increases social trust, and has a number of other benefits. They often cite Japan as their paradigmatic homogeneous society; some explicitly say they want a white version of Japan.
Those are the reasonable ones - the less reasonable ones tend to communicate in memes, threats, and slurs ("Fuck you, Jew! How about open boarders for ISRAEL!!", etc.). But the fact that these men are dedicating so much time, effort, and passion into those memes, threats and slurs says something important. It says that there is passion in this movement.
Is the alt-right really a growing, rising movement?
Much of the passion for white homogeneity seems new to me - twenty years ago, despite the existence of Nazi-type websites like Stormfront, the idea of making America an all-white nation seemed like a fringe notion. Perhaps it still is a fringe notion - after all, social media acts as a force multiplier that allows a relatively small number of highly committed individuals to seem like a huge army. And perhaps this kind of sentiment was always reasonably common in America, but simply kept under wraps by the mainstream media before the internet emerged to make it more visible.
There is some evidence to support the contention that alt-right ideas are still highly unpopular in America. A 2016 Pew survey found that only 7 percent of Americans say that growing diversity makes the country a worse place to live:
Meanwhile, recent polls find support for immigration:
So it's certainly possible that the alt-right - even defined very generally, including the more moderate "alt-light" and the quietly sympathetic "alt-white" - is a shrinking, dying idea that is only becoming louder and more aggressive because it's under threat. It's possible that Trump's election was really driven more by people's economic hopes that he would bring back dying industries and bring American jobs back from overseas, or even just by a desire to roll the dice of change.
But I think that whether or not the alt-right is really a growing, burgeoning movement, it makes sense to take it and its ideas seriously. First, the presence of Trump in the White House will probably force much of the country to listen to what the alt-right has to say. Even though he isn't really their man, he has hired several people who at least loosely sympathize with the movement's ideas - Bannon, Miller, Anton and Gorka among them. That means that at least as long as Trump's butt is planted in a chair in the Oval Office, alt-right ideas have at least a chance of making it into government policy. That means the alt-right, and their ideas, matter.
And even beyond that, I feel an emotional desire to engage with the alt-right - at least, the more reasonable among them. I couldn't care less about the people in Europe supporting Le Pen or Geert Wilders, but alt-right Americans are my countrymen. I'm a nationalist at heart and I care about what my countrymen think.
And I think that there are a decent of young (mostly) men out there whose intellectual lives will be defined by this stuff - who will spend their 20s and 30s entranced by the idea of a homogeneous white society. Just as there are old hippies who still look at the world through the lens of the 1960s anti-war movement, in a few decades there will be some aging white Millennial men for whom Pepe the Frog and r/thedonald and Kekistan and the Great Meme War were the climax of their youthful energy and imagination. I want to engage with those people, even if (as I predict) they ultimately lose.
Is the alt-right really a pro-homogeneity movement? Is Trumpism?
Every movement is...well, heterogeneous. Alt-right people talk a lot about homogeneity, but it's certainly not the only thing they talk about, or the only reason for their movement. Some may join the alt-right simply out of a fear of the social justice movement - banding together for mutual defense. Others may simply be opposed to some group of immigrants - someone who would be fine with a Cuban neighbor might be terrified of a Syrian one. Still others may be religious traditionalists looking for a home after the collapse of the Christian right, neo-Confederates allied to an Old South style of racial politics or just Trump fans looking for a cool club to join. For some, "homogeneity" might be simply a convenient rallying cry for expelling undesirable groups from the country, or for instituting one's chosen value system. As for Trumpism, that almost certainly has multiple causes - anything as big and all-encompassing as a presidential election will have multiple causes.
But I think research shows that fear of ethnic heterogeneity is a real driver of Trump support. For example, this study shows that reminding white people with strong white identification that America is getting less white (which might not actually be true, but we'll get to that later) increased support for Trump. And anecdotally, support for homogeneity pops up again and again in pro-Trump literature and discourse. Here's a quote from Trump advisor Michael Anton's famous essay "The Flight 93 Election," widely considered to be one of the basic Trumpist manifestos:
Third and most important, the ceaseless importation of Third World foreigners with no tradition of, taste for, or experience in liberty means that the electorate grows more left, more Democratic, less Republican, less republican, and less traditionally American with every cycle. As does, of course, the U.S. population[.]So I'd say the case is fairly clear that the desire for a homogeneous society runs strong through both the alt-right and the broader Trump movement.
The data-based case for homogeneity
(Note: When I talk about "homogeneity" in this post, I'm only talking about the ethnic/racial type. I'm not talking about linguistic, religious, or other dimensions of homogeneity/diversity.)
The case for homogeneity comes down to the idea that a homogeneous society is a nicer place to live. Alt-right people cite Japan's stunningly low crime rate, for example, as evidence that ethnically similar people don't fight. They also claim that homogeneity increases social trust.
There is a reasonably large body of research that supports the "trust" idea. For a good list of links to those papers, check out this post by blogger James Weidmann, better known as Roissy. Roissy sums up the thesis in one simple equation: "Diversity + Proximity = War." I'm not going to replicate the whole list here, but here's a very small sampling:
1. A study in Denmark showing a negative correlation between reported trust and ethnic diversity at the municipality leve from 1979-2009
2. A study in Britain find that people who stay in communities after those communities become more diverse report more negative attitudes toward their communities afterward
3. A study in the Netherlands finds that increasing diversity in classrooms made kids more likely to choose friends of similar ethnicity
4. A study found that across Europe, different-ethnicity immigration tends to decrease social trust, while similar-ethnicity immigration tends to increase it.
Roissy didn't include econ papers on his list, but economists have also flagged the dangers of ethnic divisions. Alesina, Baqir, and Easterly (ironically, a rather diverse team of authors) famously found that ethnic divisions reduce public good provision. Alesina, Glaeser, and Sacerdote hypothesize that diversity is what prevents America from having a Europe-style welfare state.
There are lots of postulated mechanisms for how diversity reduces trust and leads to dysfunctional societies. Maybe people are genetically programmed to cooperate with those who are genetically more similar to them. Maybe people who belong to different groups have different interests. Maybe we just generally fear that which is different and strange.
On top of this appeal to evidence, however, there's an emotional appeal - as there always is for any really important political idea. There's the negative appeal of fear of diversity - the specter of becoming a minority, potentially hated, despised, and/or oppressed by other groups. But there also seems to be a yearning for a half-imagined utopia - a "Japan for white people", where shared whiteness produces a neighborly camaraderie, social cohesion, and peace that is unknown in much of modern America.
Caveats to the data-based case for homogeneity
Roissy is a polemic blogger; his aim is to advocate, not to educate. The academic case for homogeneity is not nearly as clear-cut as what he presents.
Many of the studies he cites have methodological issues. For example, one study finds that "neighborly exchange" is negatively correlated with diversity. But its data set doesn't allow it to compare recently diversified neighborhoods with neighborhoods that recently received a lot of internal in-migration - in other words, it may simply be that a flood of newcomers, be they the same race as the majority or not, tends to disrupt neighborly friendships. In fact most of the cited studies tend to have this problem - it's hard to distinguish between the impact of population mobility and the impact of diversity itself.
Other studies he cites show some cases in which ethnic diversity increases trust. For example, a study in America found a U-shaped relationship between ethnic fractionalization and trust, meaning that high and low diversity places tend to have more trust than medium-diversity places (which makes sense if medium-diversity places are places where a bunch of newcomers just showed up).
Also, it's worth noting that many of the studies Roissy cites are from Europe. It may be the case that Europe functions differently than America, and is not an appropriate comparison. Most Europeans may think of their societies as based on ethnicity - "blood and soil", as some say - while this may hold true for only a minority of Americans. Also, recent European nonwhite immigration may be very different from the type of nonwhite immigration America gets - where America has recently mostly taken in hard-working Hispanics and high-skilled Asians and Africans, Europe has tended to take a lot of lower-skilled Middle Easterners and North Africans. Not only might the latter tend to be a more fractious type of immigrant, but there's also an enmity between Europe and the MENA region that goes back further than reliably recorded history. That could contribute to the distrust. In other words, the kind of diversity you get probably matters a lot.
Then there are all of the contrary studies Roissy, as a polemic blogger, doesn't cite. It's a big literature, and there are lots of findings that go in the other direction. For example:
1. A recent study in Southern California found that ethnic diversity is associated with decreased crime and higher home values
2. A study in Britain showed no relationship between ethnic diversity and trust.
3. A study in Europe found a positive long-term effect of diversity on trust.
4. A 2014 literature survey finds that "ethnic diversity is not related to less interethnic social cohesion."
5. A 2008 study in Europe found that ethnic diversity didn't decrease social capital.
6. A 2007 study in Britain found that the negative effect of diversity on social cohesion disappears after controlling for economic variables.
7. There's also a big literature on diversity and group decision-making, most (but not all) of which concludes that ethnic diversity makes groups smarter.
I could go on - most of this is the result of me just doing Google Scholar searches for "diversity and trust" and "diversity and social capital" and picking out any studies on the first page or two that seem to contradict the "diversity decreases trust" conclusion. That's hardly a scientific way to proceed, but it does show that if you get your academic information from a polemicist, you're going to get a distorted picture of the academic literature.
My point here is not to say that the alt-right is wrong about homogeneity and trust. They might be right - my sense from reading literature surveys is that the correlation between homogeneity and trust is a common finding, but not overwhelmingly common. My point here is to say that the question of homogeneity and trust is not yet answered. This is not surprising, because both homogeneity and trust are big, expansive, vaguely defined concepts, which usually means clear-cut answers don't exist.
Another thing that bothers me about many of these studies is that I tend to be a bit skeptical of survey research. This is not to say survey research is worthless, but I guess like any good economist I instinctively put more stock in measures of actual behavior. Roissy's link list does include some studies showing diversity increases conflict, but to my knowledge, the academic consensus is that immigration reduces crime (including in Canada). That literature review is from a few years back, but recent research all seems to confirm the finding. To me, lower crime is a much more tangible result than people simply saying negative things on a survey.
But an even more important reason why you shouldn't put too much stock in this literature is that almost none of these studies are very good at dealing with endogeneity. Here are some examples of endogeneity issues:
* Suppose low-skilled immigrants tend to move to areas with low social trust, because businesses in places with low social cohesion tend to hire cheap labor.
* Suppose large empires tend to conquer lots of different ethnicities and encourage internal migration that increases local ethnic diversity, but suppose that large empires also tend to collapse, causing lots of local conflicts.
* Suppose exogenous events that cause waves of newcomers - conflicts, recessions, out-migration from declining areas - are also things that tend to reduce trust.
To really control for these kinds of things, you really need natural experiments. They already do this for things like the impact of immigration on wages. But to isolate the effect of ethnic diversity from the effect of population mobility - i.e., to tell the difference between "newcomers of any race" and "newcomers of a minority race" - will require finding some situation where different ethnicities of newcomers are randomly assigned to different areas.
(Update: Someone forwarded me this paper showing that when housing is randomly assigned in France, diversity is correlated with "social anomie", which apparently increases vandalism but reduces violent conflict. Interesting! Keep in mind that this might be specific to the types of people who live in France.)
(Update: Bryan Caplan did some sleuthing, and found out that immigrants to America adopt American levels of social trust really quickly. So that's interesting.)
Anyway, so this is all important to think about. But to me, the really interesting question is whether ethnicity itself is endogenous.
More on that later, though. First, let's shift gears from data to anecdote, so I can talk about my experiences living in an ethnically homogeneous society.
My own experience in a homogeneous society
As regular blog readers know, I've live in Japan (for a total of about 3.5 years). Though I'm of course not Japanese, the experience taught me much about how Japanese people live and think. So I have observed at least one good example of a homogeneous society up close. While that example might not generalize, here are my thoughts.
First of all, if you think Japanese people share a sense of camaraderie and togetherness from all being the same ethnicity, think again. Because Japan is homogeneous, ethnicity just isn't that salient to most Japanese people - when a Japanese person meets another Japanese person, they don't think "Japanese person," they just think "person". Ethnic identity isn't on their minds.
Because of this, ethnic homogeneity creates very little solidarity on a day-to-day basis in Japan. Japanese people are generally wary of striking up conversations with strangers - more wary than Americans of different races are of striking up conversations with each other, I find. Services like Craigslist that facilitate informal transactions between private parties are rarely used - when I ask Japanese people why, they say it's because they can't trust strangers. Some Japanese people have told me that they feel far less shy talking to a foreigner than they do talking to another Japanese person.
I suspect that the feeling of ethnic solidarity that many alt-right whites feel for other alt-right whites is something unique to minorities. People who have always been part of the overwhelming majority just don't think about ethnicity enough for it to create bonds of solidarity - except in extreme situations, like a foreign war.
Surveys corroborate my hunch. Japan has always reported relatively low levels of interpersonal trust - until recently, considerably lower than in the U.S.:
Now keep in mind, that's trust, which is very different from trustworthiness. Japanese people, as a rule, are some of the most scrupulously honest people I've ever met. I've had old Japanese women run to catch up with me on the street, handing me a penny I dropped. The one time I dropped a substantial amount of cash on the ground, it was a yakuza bodyguard who notified me. Japanese people generally deserve high trust, but don't necessarily give it to each other.
Urban Japan also seems to me to have little tradition of "neighborly exchange" (I'm sure this is different in small towns, but Japan is very highly urbanized). I see very few people saying hello to their neighbors. One person I knew who did this was considered eccentric.
So if you think a homogeneous society means that people will tip their hat to you on the street and be you're friend just because you're the same race as them, think again.
However, Japanese culture also has quite a lot of unwritten rules, which almost everyone follows. Some of these are speech rules - the famous Japanese "politeness". Some are rules about work - the famous Japanese "corporate culture". Some are rules about service in restaurants and shops. There are many others.
These rules - which people sometimes mistakenly label "conformity" - would be harder to turn into universal norms in the diverse United States. Foreigners, or people from other parts of the country, might just not know the rules. And people from certain ethnic backgrounds might resent being pressured to follow those rules by people of other ethnic backgrounds, and so might intentionally disobey. The less other people follow a social rule, the less incentive there is for me to follow it.
So Japanese homogeneity seems to produce a society where everyone's minor, day-to-day interaction is a little more predictable.
How about politics? Japan has long been dominated by a single political party (the LDP), and politics is traditionally conducted via factions within that ruling party. There's little question in my mind that homogeneity is one of the causes of one-party dominance - there's no ethnic minority to form the core of an opposition party.
So how does that work out? Japanese politics is famously dysfunctional - the debt is out of control, patronage politics is rife, and there's usually a dearth of leadership. This was as true before World War 2 as it is today - Japan in the 30s was afflicted with frequent coup attempts and plenty of extremism, and essentially bumbled its way into multiple disastrous wars. Nowadays, Japanese political dysfunctionality is more likely to manifest itself as wasteful spending and obstruction of needed economic reforms.
However, it's worth noting that Japan has not experienced a "populist backlash" like other countries. Shinzo Abe is a true nationalist leader, and a responsible one. He was quick to quell outbursts of racism against those minorities that do exist in Japan, and in general has a pretty progressive agenda. And overall, Japanese people are (so far) pretty happy with Abe. He's worlds away from a Trump or a Le Pen or an Erdogan or a Chavez. So it's possible that homogeneity exerts a stabilizing effect on Japanese politics, insulating it from periodic outbreaks of madness, while making it less responsive in normal times due to the lack of a credible opposition.
As for crime, everyone knows that Japan is an extraordinarily safe country. It's hard for people who've never lived there to wrap their heads around how safe it is - teenage girls walk the streets of major cities alone at night in schoolgirl skirts and fear absolutely nothing. Is Japan so nonviolent because of its homogeneity? It's hard to say. In America, immigration - which is usually nonwhite immigration - tends to decrease crime. The ultra-diverse New York City and Los Angeles are some of the lowest-crime cities in America. Also, Japan does have a few very diverse neighborhoods, and these are also quite safe. So my instinct is to say that Japan's secret safety sauce is something else. But I don't really know.
So overall, if I were to draw conclusions from my experience in Japan, I'd say that homogeneity has its advantages and disadvantages, but ultimately isn't clearly better or worse. Japan is one of the awesomest, nicest places I've ever been, but the other top contenders are diverse places like Vancouver, Austin, and the San Francisco Bay Area.
(As an aside, if I were making policy, I'd recommend that Japan not take in mass immigration. Maybe their society could handle it, maybe it couldn't - but I say, no need to mess with a good thing. But that's also why I recommend that America and Canada keep taking in lots of immigrants - we've got a different kind of good thing going. Anyway, that's my instinct.)
How racial is homogeneity?
But here's one coda, which leads into my next point. Are Japanese people all the same race? Maybe not. Japan was formed from the confluence of two groups, the Jomon (unusually densely populated hunter-gatherers) and the Yayoi (rice farmers). This genetic mixing is still very apparent in the genetic data. And perhaps as a result of this, you see a reasonably large diversity of features in Japanese people. For example, here are two Japanese guys:
Are those two guys the same race? Technically, yes. In America they'd both be "Asian", in Asia they'd both be "Japanese". Neither American culture nor Japanese culture recognizes any ethnic difference between these two men. And sure, they both have straight black hair, and their skin tones aren't that different. But the pretty big difference in physical appearance between those two guys - and between many people in Japan - makes me wonder whether our definitions of race aren't a little...elastic.
What if homogeneity is a choice?
In lefty circles, it's common to hear people say that "race is a social construct." What could that possibly mean? Obviously, physical differences are real. And obviously, those differences are going to be clustered, because for most of human history - and even now, really - there was only limited population mixing across areas. A clustering algorithm will pick out clusters of traits, and you can call those "races" if you want.
But are the "races" we recognize the same that would be picked out by a clustering algorithm? Sometimes, sure. But not always. The two pictures above demonstrate that even in a supposedly super-homogeneous place like Japan, genetic differences exist that culture and society just don't recognize as representing different ethnicities.
Another important example is "Han Chinese". When you look at the genetics, Han Chinese people are actually pretty diverse. Another is "Turkish". Here are two Turkish actors I just found by Googling:
Wow. Compared to these guys, the two Japanese guys above look like twins. Obviously, these two men have ancestors from very different geographic locations, and yet somehow they're both Turks. Just like some British people have red hair and some have black, and just like some Japanese people have "sauce faces" and some have "soy faces", some Turkish people have dark skin and some have light. A difference in appearance need not translate to a difference in race, in the real world.
But the most interesting example might be "white." In America, we have a race called "white" that Europe just doesn't seem to have. In Europe, anecdotally, ethnicity is defined by language, and perhaps also by religion. While skin color differences are recognized, European ethnic definitions are usually much finer. In America, though, they're all just "white."
In fact, who's included in "white" seems to change quite a lot over time. In 1751, Benjamin Franklin was arguing against North European immigration on the grounds that Swedes, French people, Russians, and most Germans weren't "white":
Which leads me to add one Remark: That the Number of purely white People in the World is proportionably very small. All Africa is black or tawny. Asia chiefly tawny. America (exclusive of the new Comers) wholly so. And in Europe, the Spaniards, Italians, French, Russians and Swedes, are generally of what we call a swarthy Complexion; as are the Germans also, the Saxons only excepted, who with the English, make the principal Body of White People on the Face of the Earth.
What a difference two and a half centuries make, eh? And the expanding definition of whiteness doesn't seem confined to the distant past, either. Twentieth-century immigrant groups like Italians, Jews, and Poles were initially not considered "white" (except by the legal system), but rather "white ethnics". Now, no one in America questions whether Italians are white, and were in fact white all along, from the very start. And the only people who question whether Ashkenazic Jews are white are a few screeching Nazis on Twitter (who may or may not reside in the U.S.).
In fact, this may already be happening with Hispanics. More and more Hispanics are declaring themselves white.
"Black" and "Asian" are other examples. In America, "black" people are all assumed to be part of one big race, as are "Asian" people. But try telling Hutus and Tutsis in Africa that they're both part of the same ethnically homogeneous group. Or try going to a bar in Korea and telling some guys that they're the same race as Japanese people (My advice: Be ready to duck). Ethnic differences that Americans don't even recognize the existence of are the basis of genocide in other parts of the world.
"White" too. Hitler's plan for the Soviet Union involved genocide of Slavs on a scale so epic that it makes it clear the Holocaust was just a dress rehearsal:
Now that's some #whitegenocide, right there. Even though Germany lost, they made considerable headway toward making that plan a reality, slaughtering over 20 million Russians.
So you have blue-eyed Turks thinking they're the same race as black-haired Turks. You have pale Americans and swarthy Americans both calling themselves "white". And then you have Germans launching an all-out apocalyptic war to exterminate a group of people that they probably couldn't even tell from themselves if they all had the same clothes and haircuts.
(Random anecdote: One time, in Germany, a German woman came up to me and started speaking rapid German. She was astonished to find that I was American, and said "But you look so German!")
OK, but suppose you don't buy all this stuff about the social definition of race. That's hippie-dippy bullshit, right? Genetic differences are real, end of story. OK, but even then you must admit the power of intermarriage.
Intermarriage was probably essential for the creation of the white race here in America. This is from a recent National Academy of Sciences report titled "The Integration of Immigrants into American Society":
Historically, intermarriage between racial- and ethnic minority immigrants and native-born whites has been considered the ultimate proof of integration for the former and as a sign of “assimilation” (Gordon, 1964; Alba and Nee, 2003). When the rate of interethnoracial or interfaith marriage is high (e.g., between Irish Americans and non-Irish European Americans or between Protestants and Catholics), as happened by the late 20th century for the descendants of the last great immigration wave, the significance of group differences generally wanes (Alba and Nee, 2003). Intermarriage stirs the ethnic melting pot and blurs the color lines.
When tons of people have Irish, German, and English ancestors, it's just very hard to keep those three ethnic categories separate in society. The same thing happened to Italians and Jews after World War 2. In the early 1960s, the outmarriage rate among Italian Americans was over 40 percent. Jews took a little longer, but got there eventually - the Jewish outmarriage rate is now 58 percent, and among the non-Orthodox it's 71 percent.
(In case you were wondering, somewhere around 33% of native-born Hispanic and Asian Americans currently marry non-Hispanic whites.)
Whether you believe race is fundamentally about biology or sociology, intermarriage erases racial boundary lines. It's the final proof that ethnic homogeneity is not fixed, but changes depending on what people do.
An alternate theory: Trust causes homogeneity
Once you realize that homogeneity can be produced, through redefinition and through intermarriage, an alternate theory presents itself for why there might be a correlation between homogeneity and trust: Places with high trust become more homogeneous over time.
This could happen genetically. When people associate freely and don't have intergroup suspicions and hatreds, they probably tend to hook up and get married with each other a lot more. Over time, the prevalence of trust leads to a genetically homogeneous group.
This could also happen socially. When people of disparate groups are bound together for a common purpose - fighting a war against a neighboring country, for example - the increased feeling of solidarity and commonality might cause them to start to consider themselves as one single race.
So what produces trust? Perhaps another big, nebulous thing: institutions. Research shows that when organizations like the military, colleges, and public schools put people in close contact and make them cooperate, they start to trust people of other ethnic groups more. For example, here's the abstract of a 2006 American Economic Review paper called "Empathy or Antipathy? The Impact of Diversity":
Mixing across racial and ethnic lines could spur understanding or inflame tensions between groups. We find that white students at a large state university randomly assigned African American roommates in their first year were more likely to endorse affirmative action and view a diverse student body as essential for a high-quality education. They were also more likely to say they have more personal contact with, and interact more comfortably with, members of minority groups. Although sample sizes are too small to provide definitive evidence, these results suggest students become more empathetic with the social groups to which their roommates belong.
And here's the abstract from a very recent paper called "Trust, Ethnic Diversity, and Personal Contact: Experimental Field Evidence":
We combine a lab and a field experiment in the Norwegian Armed Forces to study how close personal contact with minorities affect in-group and outgroup trust. We randomly assign majority soldiers to rooms with or without ethnic minorities and use an incentivized trust game to measure trust. First, we show that close personal contact with minorities increases trust. Second, we replicate the result that individuals coming from areas with a high share of immigrants trust minorities less. Finally, the negative relationship between the share of minorities and out-group trust is reversed for soldiers who are randomly assigned to interact closely with minority soldiers. Hence, our study shows that social integration involving personal contact can reduce negative effects of ethnic diversity on trust.
Crucially, unlike most of the papers about diversity and trust cited above, these studies are randomized experiments.
Because they're randomized experiments, they're inevitably small-scale. These are moderate, short-run effects - to really know whether institutions like schools and the military can erase racial boundaries over many decades is beyond the scope of controlled experimentation. So these papers are really just suggestive.
But the notion seems to fit with American history. The Civil War seemed to put an end to the eruption of anti-Catholic sentiment, allowing Irish and South German Americans to integrate both socially and genetically into the emerging white race. And after World War 2, the outmarriage of Italian, Jewish, and Polish Americans accelerated. In both cases, the experience of being part of a nation at arms, cooperating side by side in a desperate, titanic struggle, probably erased a lot of the suspicions, prejudices, etc. that had persisted before the wars.
Anyway, this alternate theory can potentially explain the correlation between trust and homogeneity - places with institutions that create high trust levels tend to become more homogeneous over time.
An alternate theory: "War + Proximity = Diversity"
But what about all those wars? Most of the time there's a really big war, there's at least some modest ethnic difference between the combatants - British vs. French, German vs. Russian, Hutu vs. Tutsi, Japanese vs. Korean. If small differences like those could cause such incredible bloodshed, think about what calamities could be caused by the difference between groups as distinct as Africans and Europeans!
In fact, I think the historical record gives us a clue as to why this idea is wrong. The bloodiest wars in history are mostly either civil wars in China, or interstate wars in Europe or East Asia. This was true even when Europe and Japan had global reach. They chose to kill people who looked a lot like them, rather than people who looked very different. In fact, genocides between extremely distinct groups - for example, the Belgian genocide in the Congo - are the exception, not the rule. In fact, plenty of mass killings happen among people who don't recognize any ethnic differences between the sides at all - the Khmer Rouge, Mao Zedong, the Spanish Civil War, etc.
So we have big genetic differences not even being recognized in some parts of the world, and tiny, possibly undetectable genetic differences being the basis for genocide in other parts of the world. I'd say the thesis that "Diversity + Proximity = War" is, at the very least, suspiciously incomplete.
A better general theory, I think, is that most competition happens between groups of people that are pretty similar. Similar people have similar interests and desires, which naturally leads them to compete. But when people fight en masse, they need ways to organize themselves in order to motivate soldiers to kill others who look and act like them. Thus, they exaggerate any small differences they can find. "You're German, superior to those inferior Slavs; exterminate them!" Etc.
Under this theory, the "#whitegenocide" that some alt-right people fear - a term they use for race mixing - is actually the exact opposite of real genocide. Under this theory, race mixing happens when high social trust causes group differences to stop mattering, while genocide happens when low social trust causes previously insignificant group differences to start mattering.
To sum up, instead of "Diversity + Proximity = War", we might theorize that "War + Proximity = Diversity" - wars give people a reason to emphasize and magnify small differences.
It's why you don't often see humans fighting emus.
A compromise theory
Given the evidence on both sides, and the plausibility of both the pro-homogeneity and the pro-diversity theories, it seems at least somewhat likely to me that the real world features a combination of the two. Here's how the compromise theory goes: At first, when an influx of new people comes in, there's a natural reaction of distrust, and existing communities get fractured. However, as time goes on, the previous inhabitants and the newcomers get used to each other. This process is accelerated by integrating institutions like public schools, colleges, and the military, and is complete once intermarriage is widespread. However, social conflict, especially political conflict, can keep this integration from happening, causing groups not to mix and people to continue to emphasize and maintain their differences.
So the compromise theory says: In the short run, increased diversity causes decreased trust; in the long run, high trust cause increased homogeneity.
Or, as I once put it on Twitter: "One different-looking person in your neighborhood is a guest. 100 are an invasion. 1000 are just the neighbors."
Update: I should mention that this compromise theory is basically Robert Putnam's conclusion:
[E]vidence from the US suggests that in ethnically diverse neighbourhoods residents of all races tend to ‘hunker down’. Trust (even of one's own race) is lower, altruism and community cooperation rarer, friends fewer. In the long run, however, successful immigrant societies have overcome such fragmentation by creating new, cross-cutting forms of social solidarity and more encompassing identities. Illustrations of becoming comfortable with diversity are drawn from the US military, religious institutions, and earlier waves of American immigration.
If this theory is right, America's success depends on having institutions strong enough to integrate Asians and Hispanics - the two most recently arrived big groups - with the existing groups of whites and blacks. In other words, this theory says that homogeneity isn't the means, it's the goal.
Who knows; one day even white and black Americans might consider themselves part of the same ethnic group.
The dream of a white nation
But what about the people who don't want that? What about the alt-right folks and fellow-travelers who have no intention of waiting around for America's various races to all decide they're on the same team? Many want to take the shortcut to a homogeneous society - they want to live in a place where only white people are allowed. They want the dream of a half-remembered, half-imagined 1950s Southern California - the clean streets, the nice lawns, the dependable white neighbors who tip their hat and say hi to you as they stroll down the lane. And dammit, they want it now.
Well, the short answer is: I don't know how they're going to get it. It's not going to be possible for them to reimplement racial segregation, or kick all the Asians and Hispanics out of the country. Any serious, large-scale attempt to do that would mean civil war and the collapse of America, which I guarantee would not lead to a nice pleasant racially homogeneous peaceful life for anyone anytime soon.
And what are the other options for creating Whitopia? Secede? Not gonna work. You can go to small towns and gated communities, but the jobs won't follow you, and by the law of the land, any nonwhite person who wants to can buy the house next to you. So what other options are there? Move to Argentina, I guess. Or maybe New Zealand.
It's this paucity of options, I think, that has so many alt-right people so freaked out. For people who want a white heterogeneous society, there's pretty much just nowhere to go. Until recently there was Europe, but with the rise of substantial nonwhite minorities there, and with most European leaders still committed to allowing large-scale nonwhite immigration, that avenue to Whitopia - or Kekistan, as it were - seems closed down. To those who dream of white homogeneity, it must seem like they're being hounded to the ends of the earth, denied any place to call their home, told everywhere by their leaders to integrate with the nonwhite people nextdoor. No wonder they're going crazy on Twitter.
I wish it were different. I wish there were some island nation where alt-right folks could go, and establish their all-white nation-state. It doesn't seem likely to happen, but if it could, I'd say: More power to you.
But the ironic thing is, suppose they did get their Kekistan. Suppose New Zealand decided to become an all-white country (like it did in 1920), and twenty million alt-right types from around the world moved there (giving it about a quarter the population density of Japan). I think it just wouldn't work.
I think people would move there, and find that homogeneity doesn't automatically produce trust and goodwill and social peace. They would find that their population was a highly selected set - it would be made up of people who couldn't get along with the people in their homelands. And they would find that the real thing keeping most of them from getting along with their neighbors wasn't ethnic diversity - it was their own personalities.
Eventually, social strife would return. Neighbors would feud over land and resources and power and community status. Gunfights would erupt. Killdozers would be unleashed. The government would lurch from crisis to crisis. Protectionist economic policies would be tried and would fail. The economy would languish. Some people would emigrate, back to the hellscapes of diversity.
And those who remained would cling to the theory that "Diversity + Proximity = War". No one likes to give up their cherished social theories, especially if it's the theory that the country was founded on. Just as with Hutus and Tutsis, the inhabitants of Kekistan would "discover" ethnic differences that had been there all along. Suddenly they wouldn't be just white people anymore, but Russian-Kekistanis, Italian-Kekistanis, Hungarian-Kekistanis. Strife and distrust would return, and the new country would undergo decades, if not centuries, of brutal upheaval, fragmentation, clan warfare, unstable military rule, competing aristocracies, atrocities, and poverty.
I didn't just make that prediction up, by the way. That's pretty much just the history of Japan.
So although there's certainly a case to be made for homogeneity, I'd say the case is a lot weaker and more uncertain than its proponents believe. And more importantly, there's no path for how to get there - at least, not for a country like America. Except for a few small towns scattered throughout the country, the dream of an all-white utopia is likely to remain just that.