Background The complex social behavior exhibited by zebra fish is often leveraged in preclinical studies to investigate whether and how psychoactive compounds modulate inter individual interactions. Due to theoretical and methodological constraints, previous studies on the effects of ethanol (EtOH) on social behavior focused on homogeneous groups in which all individuals were treated, thereby limiting the possibility of isolating all the intervening variables.

Methods To identify how a social group affects the individual response to EtOH, we quantified the behavior of a single treated individual (acute 0.00, 0.25, 0.50, and 1.00% concentration/volume) swimming together with a group of untreated subjects or alone. A novel in‐house‐developed automated tracking system was utilized to extract the trajectories of each subject and analyze individual and social behavior. Specifically, we characterized the locomotion of each individual, the cohesion and degree of alignment of the group of untreated subjects, and the interaction between treated and untreated subjects.

Results Individual response to high EtOH concentrations varied depending on the presence or absence of conspecifics. Specifically, EtOH‐exposed subjects swam faster when group‐tested than in isolation. Remarkably, the presence of the exposed individual substantially influenced the behavior of the untreated subjects. Thus, untreated subjects swam faster when the treated individual was exposed to intermediate EtOH concentrations, without varying their cohesion and degree of alignment. No change in the distance between treated and untreated subjects was found; however, the likelihood that the swimming direction of the treated individual anticipated the response of the group was influenced by EtOH concentration.