<p>Avoiding predation is essential for most animals. For group-living species, effective predator avoidance relies on making fast and accurate collective decisions. However, the mechanisms underlying the ability to make adaptive collective decisions and to coordinate movements under predation threat remains unclear. Here, we used guppies artificially selected for divergence in the size of the telencephalon, the main brain region for advanced decision-making in vertebrates, to test the influence of telencephalon size on collective decision-making under predation threat. We measured the latency and accuracy of collective decision-making to avoid a model predator in guppy shoals. In addition, we used high-resolution tracking analysis to assess shoaling dynamics under predator threat between the telencephalon size selection lines. We found that collective decision-making latency was shorter in large telencephalon guppy shoals, indicating that variation in telencephalon size can cause variation in the ability to avoid predation. This result is unlikely to be driven by differences in boldness, as several standard tests suggest that there is no difference in boldness between the telencephalon size selection lines. General aspects of shoaling dynamics did not differ between the telencephalon size selected lines. Our study highlights that rapid mosaic changes in brain region size may be an important mechanism behind social behavioural variation with strong fitness implications.</p>