Period life expectancy is defined as the average number of additional years someone of a given age would live if current age-specific mortality rates were to stay the same for the remainder of their life. It is one of the world's most widely used population health metrics to summarize, compare and rank the mortality situation of countries, forming the basis for various public health, life insurance and retirement policies (Luy et al., 2020). This is because a country's life expectancy reflects, among other things, its existing socio-economic conditions and the quality of its public health and healthcare infrastructure (Ho & Hendi, 2018). Although immigration and emigration events are routinely factored into estimates of life expectancy, the potential impact of the unique – and typically low mortality – of international migrants (Aldridge et al., 2018; Shor & Roelfs, 2021) on national life expectancy has received little attention. A handful of studies have so far been limited to Australia (Page et al., 2007) and the United States (US) (Hendi & Ho, 2021; Mehta et al., 2016; Preston & Elo, 2014). In the context of rising shares of migrants in many countries (United Nations, 2019), alongside their “ageing in place” (Ciobanu et al., 2017), the extent to which the mortality of international migrants affects national population health demands attention.

Here, we aim to understand whether and how the mortality of international migrants affect the estimation and comparison of national life expectancy in four Nordic countries. Denmark, Finland, Norway and Sweden are all high-income countries in the latest stage of health transition. They have a long tradition of collaboration with shared features of social policy and universal welfare (Knudsen et al., 2019). Despite this, health inequality gaps remain large in the region relative to countries with less developed welfare systems (Mackenbach et al., 2016). International migration has been the major driving force behind population growth in the Nordic region in the past few decades. All four countries have experienced large increases in their absolute and relative numbers of migrants, alongside a transformation in migrant inflows from principally intra-Nordic flows to flows from all over the world (Karlsdottir et al., 2018). In recent years, life expectancy gains have slowed within the region; national life expectancies have also converged, although men and women in Denmark and men in Finland continue to lag some way behind Norway and Sweden (Knudsen et al., 2019).