[Not my area of expertise, but I would be surprised if the core thesis was wrong in a significant way. Probably not as original as I think it is. Based on a previous blog post of mine that went in a very different/weird direction. Cross-posted from Grand, Unified, Crazy.]
Currently, different human cultures have different behavioural norms around all sorts of things. These norms cover all kinds of personal and interpersonal conduct, and extend into different legal systems in countries around the globe. In politics, this is often talked about in the form of the Overton window, which is the set of political positions that are sufficiently “mainstream” in a given culture to be considered electable. Unsurprisingly, different cultures have different Overton windows. For example, Norway and the United States currently have Overton windows that tend to overlap on some policies (the punishment of theft) but perhaps not on others (social welfare).
Shared norms and a stable, well-defined Overton window are important for the stable functioning of society, since they provide the implicit contract and social fabric on which everything else operates. But what exactly is the scope of a “society” for which that is true? We just talked about the differences between Norway and the U.S., but in a very real sense, Norway and the U.S. share “western culture” when placed in comparison with Iran or North Korea. In the other direction, there are many distinct cultures entirely within the U.S. with different norms around things like gun control. The categories were made for man, not man for the categories.
However blurry these lines are, it might be tempting to assume that they get drawn roughly according to geography; it's certainly reflected in our language (note my use of "western culture" already in this post). But this isn't quite right: the key factor is actually interactional proximity; it's just that in a historical setting geographical and interactional proximity were the same thing. Time for an example.
Ooms and Looms
Back in the neolithic era, the tribe of Oom and the tribe of Loom occupied opposite banks of their local river. These two tribes were closely linked in every aspect: geographically, linguistically, economically, and of course, culturally. Because the Ooms and the Looms were forced into interaction on such a regular basis, it was functionally necessary that they shared the same cultural norms in broad strokes. There was still room for minor differences of course, but if one tribe started believing in ritual murder and the other didn't, that was a short path to disagreement and conflict.
Of course, neolithic tribes sometimes migrated, which is what happened a short time later when the tribe of Pa moved into the region from a distant valley. Compared to the Ooms and the Looms, the Pas were practically alien: they had different customs, different beliefs, and spoke a different language altogether. Unsurprisingly, a great deal of conflict resulted. One day an amorous Oomite threw a walnut towards a Pa, which was of course a common courting ritual among both the Ooms and the Looms. Unfortunately, the Pa saw it as an act of aggression. War quickly followed.
Ultimately, the poor Pa were outnumbered and mostly wiped out. The remaining Pa were assimilated into the culture of their new neighbours, though a few Pa words stuck around in the local dialect. Neolithic life went on.
In this kind of setting, you could predict cultural similarity between two people or groups purely based on geographic proximity. It was possible to have two very different peoples living side by side, but this was ultimately unstable. In the long run, such things resulted in conflict, assimilation, or at best a gradual homogenization as memes were exchanged and selected. But imagine an only-slightly-different world where the river between the Ooms and the Looms was uncrossable; we would have no reason to believe that Oom culture and Loom culture would look anything alike in this case. The law that describes this is the law of cultural proximity:
In the long run, the similarity between two cultures is proportional to the frequency with which they interact.
More First Contact
Hopefully the law of cultural proximity was fairly self-evident in the original world of neolithic tribes. But over time, trade and technology started playing an increasing role in people's lives. The neolithic world was simple because interactions between cultures were heavily mediated by geographic proximity, but the advent of long-distance trade started to wear away at that principle. Ooms would travel to distant lands, and they wouldn’t just carry home goods; they would carry snippets of culture too. Suddenly cultures separated by great distances could interact more directly, even if only infrequently. Innovations in transportation (roads, ship design, etc) made travel easier and further increased the level of interaction.
This gradual connecting of the world led to a substantial number of conflicts between distant cultures that wouldn’t have even know about each other in a previous age. The Ooms and the Looms eventually ran into their neighbour the Dooms, who conquered and assimilated them both in order to control their supply of cocoa. The victor of successive conflicts, the Dooms formed an empire, developed new technologies, and expanded their reach even farther afield. On the other side of a once-uncrossable sea, the Dooms met the Petys; they interacted infrequently at first, but over time their cultures homogenized until they were basically indistinguishable from each other.
The Great Connecting
Now fast-forward to modern day and take note of the technical innovations of the last two centuries: the telegraph, the airplane, the radio, the television, the internet. While the prior millennia saw a gradual connecting of the world’s cultures, the last two hundred years have seen a massive step change: the great connecting. On any computer or phone today, I can easily interact with people from one hundred different countries around the globe. Past technologies metaphorically shrank the physical distance between cultures; the internet eliminates that distance entirely.
But now remember the law of cultural proximity: the similarity between two cultures is proportional to the frequency with which they interact. This law still holds, over the long run. However the internet is new, and the long run is long. We are currently living in a world where wildly different cultures are interacting on an incredibly regular basis via the internet. The result should not be a surprise.
[This section is much more speculative and less confident than the rest.]
The implications for the future are... interesting. It seems inevitable that in a couple of generations the world will have a much smaller range of cultures than it does today. The process of getting there will be difficult, and sometimes violent, but the result will be a more peaceful planet with fewer international disagreements or "culture wars". A unified world culture also seems likely to make a unified world government possible. Whether the UN or some other body takes on this role, I expect something in that space to grow increasingly powerful.
While a stable world government seems like it would be nice, homogeneity has its pitfalls. There's a reason we care about ecological diversity so much.
Of course in the farther future, human culture will fragment again as it spreads into space. The speed of light is a hard limit, and while our first Martian colony will likely stay fairly connected to Earth, our first extra-solar colony will be isolated by sheer distance and will be able to forge its own cultural path. Beyond that, only time will tell.