106 K. Collins parents can find themselves divided by nationality from the individuals who are charged, ethically and legally, with their care. As Bauböck and Titshaw observe, in an era of voluntary and compelled migration, ius sanguinis is the most effective method of protecting against such destabilising and precarious circumstances. How to modernise? I agree with Titshaw and Bauböck that the modernisation of ius sanguinis citizenship, rather than its complete repudiation, offers a better way to address the problems Dumbrava identifies. The difficult question is how? I am hesitant to embrace Titshaw’s proposed method of modernisation, and I offer a friendly but important amendment to Bauböck’s proposal. Titshaw argues that the officials who administer citizenship law should adhere to the parentage determinations made by officials who generally administer family law. In the U.S., these are state-level family law judges applying state law. But domestic family law, in the U.S. and elsewhere, does not necessarily generate ideal or even tolerable outcomes on questions of citizenship. Titshaw holds up a particularly poorly drawn U.S. federal policy that regulates ius sanguinis citizenship as it applies to foreign-born children conceived using ART, but there are many examples of how the use of state family law to regulate citizenship transmission has generated equally objectionable outcomes. 5 Alternatively, Bauböck would have us adopt a ‘ius filiationis’ standard that recognises the ‘social parent’ or the ‘primary caregiver’ as the parent for purposes of ius sanguinis citizenship. He urges that this would help remedy the ‘mismatch between biologically determined citizenship and parental care arrangements that would also open the door to abusive claims’. He is correct. My concern, however, is that his emphasis on ‘social parenting’ and ‘primary caregiving’ is insufficient and has its own perils. First, it could 5 For example, in 1940 the federal ius sanguinis citizenship statute was amended to include the nonmarital children of U.S. citizen fathers under certain circumstances, such as when the father had ‘legitimated’ the child. Federal officials turned to the law of the father’s domiciliary state to determine whether legitimation had, in fact, occurred. In the 1940s and 50s, marriage to the child’s mother was a very common mode of legitimation, but federal officials making citizenship determinations would not recognise an interracial marriage as the basis of a child’s citizenship claim if the father’s home state banned such marriages – and many did. See Collins, ‘Illegitimate Borders’, above n. 2, at 2210.