People like and trust their ingroups more than outgroups - that is, people show an intergroup bias. In a diversifying society, the number of outgroups that people encounter increases. People's intergroup bias combined with a higher number of outgroups in society might result in decreased overall population trustworthiness. In five experiments (N_total = 1.489) using a minimal group paradigm, we did not find the expected decrease in overall population trustworthiness in more (vs. less) diverse populations. However, we found a robust asymmetry in people's intergroup bias, leading to equally high overall population trustworthiness. In populations of high and low diversity, people perceived their ingroup to be more trustworthy than outgroups. This effect was stronger in a population of high diversity driven by the particularly high perceived trustworthiness of the ingroup. We demonstrate that this asymmetry is not driven by the actual or perceived size of ingroups and outgroups or the subjective experience of group size in the experimental design. Rather, in a highly diverse population, it appears that higher perceived similarity between people and the ingroup explains the increase in perceived trustworthiness of the ingroup and, thus, the asymmetric intergroup bias that overshadows differences in overall population trustworthiness.