Classic arguments against recognising dual citizenship may be divided into three groups. The first group is directed at the question of whether dual citizenship is permissible under (international) law. Another group of arguments relates to technical difficulties and the third group includes socio-political objections to the concept of dual citizenship per se.
Some of the objections deriving from the above groups are explained in more detail below.
International law and dual citizenship
The most important international treaties on dual nationality are the 1963 "Convention on the Reduction of Cases of Multiple Nationality and Military Obligations in Cases of Multiple Nationality" and the 1997 "European Convention on Nationality".
According to its preamble, the 1963 convention finds its basis in the concern that "cases of multiple nationality are liable to cause difficulties and that joint action to reduce as far as possible the number of cases of multiple nationality, as between member States, corresponds to the aims of the Council of Europe". Since, however, only a few states became members of the convention, it never had any particular practical significance. In 1977 and 1993, furthermore, two protocols were added to the convention, which no longer aimed to abolish dual citizenship. In any case, in 2002 Germany was the first country to make use of the option to denounce the treaty and since that time has no longer been a member of the convention.
Instead, since 2005 Germany has been a signatory to the above-mentioned "European Convention on Nationality" which expressly recognises dual nationality. It even determines that renunciation or loss of the former nationality should not be a condition of naturalisation if such renunciation is not acceptable in the other country (Art. 14-16).
For these reasons there is now consensus that recognition of dual citizenship is not a problem under international law.
Technical objections to dual citizenship
The technical concerns expressed against multiple citizenship are based first and foremost on possible conflicts that may arise from military and tax obligations, choice of law, and confusion with regard to rights to diplomatic protection.
(a) Dual military obligations
A point of criticism previously levelled against dual citizenship, concerning the risk that people might be doubly obliged to complete national service, is nowadays no longer at the centre of debate. This is firstly due to the fact that there is a recognisable trend in most states towards abolishing general conscription.
(b) Citizenship as the basis to determine applicable law
Under international law, nationality is one of the criteria to determine the applicable domestic law, especially in the area of family and inheritance law. However, according to private international law – i.e. the principles which decide which domestic law applies in a given situation – the principle of effective citizenship means that the applicable law is that of the country to which those concerned have an effective tie, in other words the country where they normally reside.
(c) Possible double taxation of dual nationals
A state can tax its nationals regardless of their place of residence. Further, people must regularly meet their tax obligation in the country where they are economically active. This obviously gives rise to the risk of double taxation. This is, however, to a large extent insignificant
(d) Diplomatic protection for persons with dual nationality
Another technical objection relates to claiming diplomatic protection, whereby a state is entitled to protect its subjects against acts contrary to international law committed by another state. In the case of dual nationals, there could be some dispute as to which state may provide legal protection. On the other hand, conflict may arise if a state intervenes on behalf of a citizen residing in another state of which he or she is also a citizen.
Apart from the fact that experience has shown that states do not clash on account of overlapping consular rights and obligations, the International Court of Justice decided as early as 1955 that, in order for a State to exercise diplomatic protection, in addition to the formal citizenship status, there must be a genuine link with that State.
Just a hundred years ago, the second concern, i.e., state intervention within another state, was a decisive inducement to take action against dual citizenship. According to international treaties concluded since 1930, and according to the rulings of international tribunals, diplomatic protection cannot be exercised by one State against another of which the person concerned is also a national.
Socio-political objections against dual citizenship
(a) 'Unjustified' double voting rights
In nearly all states, voting rights are given on the basis of citizenship. Some commentators, therefore, critically point out that persons who are the citizens of two countries can also vote in two countries, whereas the voting rights of people with solely German citizenship are limited to Germany. Critics argue that this undermines the basic principle of equality of citizens, which is expressed by "one person, one vote".
In this regard it should firstly be noted that, generally, voter turnout amongst the overseas electorate is low. Moreover, many countries of origin have neither established a system of postal ballots, nor do they facilitate voting in their diplomatic missions.
On a theoretical level, the response to the objection that citizens' equality is being infringed may be that the frame of reference for the principle of electoral equality is the single State.
A significant objection to dual nationality lies in the assumption that it restricts the integration of dual citizens, as they do not fully identify with the country of immigration.
This objection may be responded to on four points. First, states can bar people who do not wish to accept their values and culture from obtaining citizenship. Thus German naturalisation requirements since 2007 include knowledge of the German social system, culture, history and language.
Second, there are no empirical studies that indicate that retention of another citizenship would erode any existing integration; there are no empirical findings to support those social-psychological assumptions which claim that, for example, an Afghan-German cannot be fully committed to Germany because he possesses two citizenships. Research on transnationalism is only gradually beginning to put forth better hypotheses as to how transnational activities and the feeling of belonging change during the course of a migrant's life, and how this varies between individuals and groups.
Third, making it compulsory for German-born people of foreign origin to opt for one of the two nationalities between the age of 18 and 23 may be understood by some as a signal that they are expected to be "just German" and that "Germany" does not recognise their mixed identity, despite their acknowledgement of German values. Such a perception can only have a negative effect on integration.
Fourth and last, an argument against the supposed adverse effect of dual citizenship on integration is: recognizing multiple citizenship creates an incentive for naturalisation. No one would maintain that granting citizenship – with or without the retention of another nationality – will inevitably lead to the integration of the new citizen. Yet, it is certainly a safe assumption that the granting of citizenship will simplify and improve the integration of those who would not otherwise apply for naturalisation. Even if there are few empirical studies on the outcome of these status passages, it can be expected that, due to increased political rights and formal belonging, naturalisation would lead to better positioning within, and interaction with, the majority population.
One of the main objections to dual citizenship lies in a suspected conflict of loyalties. Here, a distinction may be made between conflicts specifically mentioned and less explicit general doubts about the necessary degree of loyalty. One such possible specific conflict is that in the event of war a state depends on the undivided loyalty of the nationals it can call to arms in its service. Further, it is assumed that participation in a country´s political life – as a voter or office-bearer – could be adversely affected by divided loyalties.
With regard to the first objection, it should be pointed out that wars involving the mass drafting of civilians in countries such as Germany are unlikely. Like most modern armies, the German armed forces are constantly developing in the direction of having a smaller body of specialists, so that the non-availability of dual citizens for mass conscription in the unlikely event of war would not compromise the ability of a country such as Germany to defend itself. This would in any case only affect dual nationals from a country with which there was armed conflict.
Proponents and opponents of dual citizenship, meanwhile, agree that persons holding important public offices should give up their second citizenship.
Moreover, it should be borne in mind that not allowing dual citizenship is no guarantee that the population in Germany will consist only of loyal mono-nationals. Rather, a comparison must be drawn with the reality that, for decades and generations, millions of people have been living in Germany with just one –non-German – citizenship. These people will remain on German soil in the future, too. What are we to make of the fact that this section of permanent residents has no formal ties of loyalty with the state in which it resides?