IN TERMS of diversity, the German squad that will travel to France for the UEFA football championship next month leads the national trend. About one in five Germans has a so-called “migration background”, but nearly half the national team does. The names on the jerseys include Boateng (Ghanaian), Mustafi (Albanian via Macedonia), Bellarabi (Moroccan), and Khedira (Tunisian). Even some of the German names belie foreign origins. Antonio Rüdiger’s mother is from Sierra Leone. Bernd Leno has Russian roots.

When Germans talk about ethnic origins, they contrast the term “migration background” with its cheeky antonym, bio-deutsch (“organic German”). This reflects attitudes toward nationality that are both controversial and in flux. By tradition, Germanness has always been an ethnic identity, based on shared descent or “blood”. But today Germany is becoming a multi-ethnic society like other Western countries. This raises the question of how the state should officially treat the categories of “bio” and “migrant”.

The pressing issue this year is how to count them. Since 1957 Germany has conducted an annual “micro-census” of 1% of the population. A new law in 2005 added a complex tangle of questions every fourth year meant to determine whether a household has foreign origins. Bureaucrats decide whether to assign the label “migration background” based on the questionnaire, which has become controversial. Because the law expires this year, moreover, a new version is now wending its way through the committees of the Bundestag. This has scholars and policymakers pondering how identity should be measured.

The rest of Europe takes wildly differing approaches. At one extreme is France, which has traditionally defined citizenship as a choice. It thus officially ignores descent and ethnicity in its census questions. This was always a bit hypocritical, says Patrick Simon of the National Institute for Demographic Studies in Paris; the French used racial categories in their empire and, when it ended in the 1950s, began applying them to the ex-colonials who came to France. But they soon reverted to treating nationality as a binary matter of having or not having citizenship, regardless of descent.

Britain in 1991 and Ireland in 2010 took a different path. Like other English-speaking countries, they accepted that their societies were already multi-ethnic. Policymakers wanted to compile statistics for different groups in the hope that this might help them monitor discrimination. So they included questions about ethnicity in their censuses. But respondents self-identify by choosing from a menu of options, including “black British” and “mixed”. Ethnicity is subjective, not scientific.

The countries of eastern Europe reflect yet another tradition. They are young states that broke out of collapsing multi-ethnic empires. Their emphasis is therefore on counting centuries-old national minorities, rather than new migrants. Hungary, for example, has a list of official ethnicities, from Magyar and Slovenian to Serb or Roma. But these countries also use self-identification. One drawback is that some groups, especially Roma, may prefer not to “out” themselves for fear of stigmatisation, and thus go undercounted, says Linda Supik, an ethnologist in Essen.

Germany, the Netherlands and the Scandinavian countries form a final bloc. In the past, these societies were relatively homogenous. Today, they have a well-intentioned reluctance to admit the existence of an ethnic mainstream, which could imply that other groups are second-class citizens. Their censuses do not ask directly about ethnicity, but use proxies—such as the birthplace of parents—and then assign a seemingly objective category.

In the German case this approach harks back awkwardly to old notions of blood identity, argues Anne-Kathrin Will at the Otto-von-Guericke University in Magdeburg. To have a “migrant background”, it is enough for one parent to be born abroad. And the German system, unlike the Dutch, passes the status on to the children. Many Germans with just one foreign-born grandparent are classified as having a migrant background. But parents or grandparents who migrated to Germany before the 1950s are excluded, in order to exempt the millions of ethnic Germans who fled from eastern Europe just after the second world war. Although they were migrants, they and their progeny are not considered to have a “migration background”—in effect, they are deemed bio-Deutsche.

All approaches have their problems. The French notion that the state should deliberately ignore the ethnicity of its citizens is naive, says Mr Simon. Refusing to collect data on how many citizens have African or Arab backgrounds makes it much harder for policymakers to identify or fight discrimination. By contrast, the problem with self-identification is that categories are subjective and culturally fluid. This makes it hard to compare data over time.

But most scholars think that bureaucratic decisions made through pseudo-objective proxy questions are the worst option. When parental birth is the only criterion, ethnic information is lost with each generation. A fourth-generation German citizen who is black may want to identify with his ethnic group—as many Latino Americans do, for example. By contrast, a German child who has one foreign grandparent may not view ethnicity as relevant to her identity at all. The ultimate challenge, says Mr Simon, is to rethink the meaning of “mainstream”. The aim is to make society more cohesive. Germany’s classification system seems to be dividing it.