“…We here probe into the widely held assumption that previous language experience qua bilingual or multilingual upbringing fosters the acquisition of additional languages. Such a 'multilingual boost' (Berthele and Udry, 2019) 1 could be evidenced for balanced bilinguals acquiring English, especially in the Basque Country and Catalunya (Agustín-Llach, 2019;Cenoz, 2003Cenoz, , 2013Cenoz and Valencia, 1994;Lasagabaster, 1998Lasagabaster, , 2001Safont Jordà, 2003;Sagasta Errasti, 2003;Sanz, 2000), and more recently also for bilingual heritage speakers in their acquisition of English, most prominently in the context of Germany but partially also other countries (Edele, Kempert and Schotte, 2018;Fleckenstein, Möller and Baumert, 2018;Göbel, Rauch and Vieluf, 2011;Hirosh and Degani, 2018;Hopp et al, 2019;Klieme et al, 2006;Maluch et al, 2015;Maluch, Neumann and Kempert, 2016;Sanders and Meijers, 1995;Schwartz et al, 2007;Siemund and Mueller, 2020;van Gelderen et al, 2003). 2 It is especially this latter context of bilingual heritage speakers that we are interested in here.…”