Selection bias obfuscates the discovery of fast radio burst sources

FRBs are a newly discovered class of extragalactic radio transients characterised by their high energy and short-duration (~$\mu$s-ms)[1]. Their elusive physical origin remains a subject of ongoing research, with magnetars emerging as leading candidates[2],[3]. Previous studies have employed various methodologies to address the FRB origin problem, 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 counterparts[10]. However, these studies are susceptible to significant 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 likely 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 speculated[6]. 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 likely responsible for the observed bias[11],[12]. However, a larger sample of localised FRBs is required to robustly quantify its contribution in the inclination-related selection bias.