Selection bias obfuscates the discovery of fast radio burst sources

Fast radio bursts (FRBs) are a newly discovered class of extragalactic radio transients characterized by their high energy and short duration (from microseconds to milliseconds)¹. The physical origin of these FRBs remains unknown and is a subject of ongoing research, with magnetars emerging as leading candidates^2,3. Previous studies have used various methodologies to address the problem of FRB origin, including demographic analyses of FRB host galaxies and their local environments^4-6, assessments of FRB rate evolution with redshift^7-9 and searches for proposed multi-messenger FRB counterparts¹⁰. However, these studies are susceptible to substantial biases stemming from unaccounted radio and optical selection effects. Here we present empirical evidence for a substantial selection bias against detecting FRBs in galaxies with large inclination angles (edge-on) using a sample of hosts identified for FRBs discovered by untargeted surveys. This inclination-related bias probably leads to a significant underestimation (by about a factor of two) of the FRB rates reported in the literature and disfavours globular clusters as the dominant origin of FRB sources, as previously speculated⁶. These conclusions have important implications for FRB progenitor models and targeted FRB follow-up strategies. We further investigate the impact of this bias on the relative rate of FRBs in different host environments. Our analysis suggests that scattering in FRB hosts is probably responsible for the observed bias^11,12. However, a larger sample of localized FRBs is required to robustly quantify the contribution of scattering to the inclination-related selection bias.