Luuk van der Baaren on dual citizenship impede persons from acquiring another EU citizenship. This possibility can be further assessed on the basis of the naturalisation rate, defined as the ratio between the number of persons who acquired the citizenship of a country during a calendar year and the stock of foreign residents in the same country at the beginning of the year.25 An overview of the naturalisation rates of citizens of EU Member States in the United Kingdom as well as British citizens in EU Member States is depicted in Table 3. The overview indicates that naturalisation rates are indeed lower for citizens of restrictive states. For EU citizens who acquired British citizenship, the naturalisation rate for permissive constellations is 1.54%, while the naturalisation rate for restrictive constellations is 0.55%. For British citizens who acquired another EU citizenship, the naturalisation rate for permissive constellations is 2.94%, while the rate for restrictive constellations is 0.39%. It must be noted that naturalisation rates are an imperfect tool of measurement in this regard, in particular due to data discrepancies.26 Nevertheless, the vast difference between the naturalisation rates for permissive constellations and restrictive constellations could indicate that restrictions on dual citizenship indeed deter third-country citizens from acquiring citizenship – a finding that would resonate with earlier studies on this topic.27 The practical consequences of dual citizenship restrictions therefore deserve to be a topic of further scholarly attention. 3. Developments regarding dual citizenship in EU Member States (20102020) Austria On 11 December 2018, the Austrian Constitutional Court delivered a judgment considering Austrian-Turkish dual citizens. Through a leaked extract of the Turkish electoral register, the Austrian authorities had learned that a number of Austrian citizens of Turkish descent had retained or reacquired Turkish citizenship and consequentially had lost Austrian citizenship, which the affected persons contested in court.28 The Constitutional Court granted the appeal on procedural grounds, as it ruled that the above-mentioned extract of the Turkish electoral register could not be regarded as legal evidence and that the burden of proof in this regard rests on the Austrian authorities.29 This once again underlines that it can be difficult for states to enforce restrictive provisions regarding dual citizenship as long as states do not exchange information in this regard. Austrian citizenship law has traditionally held a negative attitude towards dual citizenship for emigrants as well as immigrants, but also provides for a small number of 25 European Commission, “Glossary: Naturalisation Rate”, Eurostat, 2014, via: <https://ec.europa.eu/>, last accessed on 20 June 2020. 26 European Commission, “Acquisition and loss of citizenship (migr_acqn)”, Eurostat Metadata File, Eurostat, 2020, via: via: <https://ec.europa.eu/>, last accessed on 20 June 2020. 27 See: Alarian, H., and Wallace Goodman, S., “Dual citizenship allowance and migration flow: an origin story”, Comparative Political Studies, Vol. 50 No. 1, 2017, pp. 133-167. 28 Case E 3717/2018-42, Constitutional Court, 11 December 2018, paras. 1-14. 29 Ibid., paras. 57-78. RSCAS/GLOBALCIT-Comp. 2020/4 - © 2020 Authors 9