Asymmetrical social copying drives behavioural cascades following a shock event.

When facing disruptive events, such as epidemics, terrorist attacks or financial crises, individuals often exhibit protective behaviours, including mass escape movements and stockpiling essential goods. These collective responses are predominantly driven by a social amplification process, where an initial decision-maker is imitated by others, triggering large-scale behavioural cascades. However, the mechanisms shaping this phenomenon remain unclear. Here, we present an experiment examining the propagation of protective behaviours in dyadic transmissions involving 210 participants. Our research uncovers an asymmetric social response, where individuals tend to copy protective behaviours more strongly than inaction. This pattern causes large amplification cascades in simulated transmission chains and is more pronounced following a brief but intense shock compared to a longer, minor one. Furthermore, our results demonstrate an extended period of collective vigilance post-shock, suggesting that social transmission can facilitate rapid behavioural adjustment during crises. By examining the interplay between individual protective behaviours and group dynamics, our study contributes to a better understanding of how adaptive social responses emerge in uncertain and dynamic environments. This article is part of the theme issue 'The evolution of collective intelligence'.