Whilst reflexive migration studies have criticised the use of categories such as ‘nationality’ and ‘second generation’ in quantitative research, several gaps on how to develop such reflexivity remain. In qualitative data, the co-construction of knowledge seems feasible during fieldwork, whereas the deductive process of quantitative research limits such interactions and is more at risk of reproducing a ‘state thought’. Through a longitudinal database, the LIVES-FORS cohort survey of the National Center of Competence in Research LIVES – Overcoming Vulnerability: Life Course Perspectives and FORS – the Swiss Centre of Expertise in Social Sciences (herafter LCS), we engage in this discussion and provide some answers. The LCS is an annual longitudinal survey that, in 2013, started following a cohort of young adults born between 1988 and 1997 who grew up in Switzerland. The underlying hypothesis of the LCS is that migrants’ descendants have access to different resources (and often a lack thereof) to Swiss natives. In this paper, we discuss both the theoretical and empirical challenges to using the categories ‘nationality’ and ‘second generation’. We show the fluidity and subjectivity of these categories. By changing the definition of the category ‘second generation’, we increased the proportion of ‘second-generation’ participants from 43 to almost 62% of the sample. Looking across the five waves of the survey, we notice a 2% unexplained variation in the first nationality mentioned by the participants and 31% missing values regarding the nationality at birth – which are both indicators that nationality is a subjective category as well as a legal one. We illustrate that the static and neutral conceptions of these categories reproduce a false and stigmatised image of migrant descendants. To avoid these pitfalls we suggest developing multilevel geographical comparisons to consider the effects of time (age and historical), to use a wider range of information in order to be more precise, to examine different nationalities instead of focusing on the traditional nationalities of labour immigrants in a given country and to explore the reasons for the lack of answers to certain questions. Thus the questionnaires should include both more flexibility in the possibilities for answers and details and more-open questions regarding sensitive issues about the definition of the self. They should be developed through a participative and bottom-up process fostering mixed methods.