By DAVID BROOKS
Let's say you are an 18-year-old kid with a really big brain. You're trying to figure out which field of study you should devote your life to, so you can understand the forces that will be shaping history for decades to come.
Go into the field that barely exists: cultural geography. Study why and how people cluster, why certain national traits endure over centuries, why certain cultures embrace technology and economic growth and others resist them.
This is the line of inquiry that is now impolite to pursue. The gospel of multiculturalism preaches that all groups and cultures are equally wonderful. There are a certain number of close-minded thugs, especially on university campuses, who accuse anybody who asks intelligent questions about groups and enduring traits of being racist or sexist. The economists and scientists tend to assume that material factors drive history - resources and brain chemistry - because that's what they can measure and count.
But none of this helps explain a crucial feature of our time: while global economies are converging, cultures are diverging, and the widening cultural differences are leading us into a period of conflict, inequality and segmentation.
Not long ago, people said that globalization and the revolution in communications technology would bring us all together. But the opposite is true. People are taking advantage of freedom and technology to create new groups and cultural zones. Old national identities and behavior patterns are proving surprisingly durable. People are moving into self-segregating communities with people like themselves, and building invisible and sometimes visible barriers to keep strangers out.
Not long ago, many people worked on farms or in factories, so they had similar lifestyles. But now the economy rewards specialization, so workplaces and lifestyles diverge. The military and civilian cultures diverge. In the political world, Democrats and Republicans seem to live on different planets.
Meanwhile, if you look around the world you see how often events are driven by groups that reject the globalized culture. Islamic extremists reject the modern cultures of Europe, and have created a hyperaggressive fantasy version of traditional Islamic purity. In a much different and less violent way, some American Jews have moved to Hebron and become hyper-Zionists.
The members of these and many other groups didn't inherit their identities. They took advantage of modernity, affluence and freedom to become practitioners of a do-it-yourself tribalism. They are part of a great reshuffling of identities, and the creation of new, often more rigid groupings. They have the zeal of converts.
Meanwhile, transnational dreams like European unification and Arab unity falter, and behavior patterns across nations diverge. For example, fertility rates between countries like the U.S. and Canada are diverging. Work habits between the U.S. and Europe are diverging. Global inequality widens as some nations with certain cultural traits prosper and others with other traits don't.
If you are 18 and you've got that big brain, the whole field of cultural geography is waiting for you.