What does it mean to be an American? For that matter, what does it mean to be Japanese, Hungarian or Australian? All around the world, how we define our national identity is complicated and often fluid. However, you may be surprised to find how different countries often have similar measures of who is "one of us" and who is not.

On Wednesday afternoon, Pew Research Center released a study that looked at how national identity is defined across 14 different countries using survey data taken at the start of last year. In light of the ongoing debate about immigration in pretty much every part of the world, it makes for illustrative reading.

It turns out, for example, that most Americans don't believe that where someone is born really defines whether they can be American or not. In fact, only a handful of the countries Pew surveyed thought this was important. And while America is a country well-known for its talk of values and God, most Americans don't think that customs and religion are really important to being an American — and neither do most other countries.

Instead, Pew's study found that in every country its researchers looked at, language was what really bound its national identity. The highest result was found in the Netherlands, where more than 84 percent of the population believes it is vital to speak Dutch if you want to truly be Dutch. But in all countries, a majority said it was "very important" to speak the national language.

The idea that language binds a nation together isn't surprising. Two people will struggle to find anything in common if they can't exchange information easily. As British historian Eric Hobsbawm wrote in a study of language, culture and national identity, as countries gradually embraced democracy, a shared language became a necessity. "The original case for a standard language was entirely democratic, not cultural," Hobsbawm wrote in 1996.

Many modern states developed around linguistic communities — the German language was one key factor in the eventual unification of Germany, for example. In the modern age, some languages have become symbols for independence movements, such as the Basque language in Spain.

Pew's study does not include countries where a multitude of languages are widely spoken — such as India or Switzerland. Instead, most of the countries listed in the research are pretty clearly defined by their language, which in many cases is unique to that country.

The most obvious countries where that is not true are both in North America — Canada and the United States. Canada is a country where there are two national languages, and both of them originated not in Canada but back in Europe. This may be why it has one of the lowest percentages of people who think language is "very important" to national identity. (It also has the highest percentage for those who think that language is "not at all important," at 5 percent.)

However, it's worth noting that Italy, a nation with deep ties to its language, has results somewhat similar to Canada's, so perhaps the relationship is a little more complicated than that. Also worth considering is the fact that the United States — another country that imported its de facto national language from Europe — seems to view the importance of language more strongly than some European nations.

Pew's data shows that beliefs about national identity have a partisan split. Eighty-three percent of Republicans say that being able to speak English is very important to being truly American, 22 percentage points higher than Democrats. There are similar splits regarding the importance of American customs and Christianity, too, though both Republicans and Democrats attach relatively low importance to being born in the United States.

This partisan link is also important across Europe, where respondents with positive views of anti-establishment parties such as the United Kingdom Independence Party (UKIP) and France's National Front were found to attach a higher level of importance to national customs in defining identity. There were also similar partisan splits in Canada and Australia (though information for Japan was not available).

In most of the countries surveyed, there is already some kind of language requirement for citizenship. In fact, citizenship tests have been becoming harsher over recent years as increasingly obscure cultural questions have begun appearing — potential British citizens are even quizzed about Rudyard Kipling. Some academics say the shifting tests are often a response to right-wing political pressure and serve little practical purpose.

But things may change. For one thing, immigration also influences language: Germany has developed a colloquial language, "Kiezdeutsch," which is primarily used by German speakers whose native tongue is Turkish or Arabic. Additionally, Pew's data suggests that there is a big generational divide on whether language is very important for identity in most countries. In America, that shift is especially pronounced: While 81 percent of those age 50 or older say language is very important to national identity, only 58 percent of those age 18 to 34 agree.

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