(A) Top view of double T-maze apparatus showing two animals interacting during social decision-making. (B) Setup diagram; in each T-maze, nose-pokes are positioned on either side of the central choice area. Sliding pneumatic doors give access to the side arms of each maze (top and bottom in diagram) where pellet dispensers deliver food rewards. Six IR beams (depicted as grey and red circles connected by a dotted red line) detect the position of the animals to safely close the doors once access to an arm is secured. (C) Focal animal individual training showing the number of trials completed per minute (left panel) and side bias (right panel) across days of training. (D) As (C) but for the recipient animal. (E) Social decision-making task. The trial starts with both animals in the central arm. The recipient animal has learnt in previous individual training to poke the port on the upper side of the diagram to give access to a food pellet in the corresponding reward area. During the social task, the recipient animal’s ports no longer control the doors but the animal can display food-seeking behaviour by repeatedly poking the previously trained port. The focal animal has previously learned in individual training to collect food from the reward areas on both sides (top and bottom of diagram) by poking the corresponding port in the central choice area to activate the doors. During social decision-making, the focal animal can either choose the ‘prosocial’ port, giving both animals access to the side (upper on diagram) of their respective mazes where both receive reward, or can choose the ‘selfish’ port, giving both animals access to the other side (lower on diagram) where only the focal animal receives reward. (F) Raster plot showing behaviour of a pair of animals over one session during early social testing. Nose-pokes are represented by vertical lines, and colour coded according to the role of each mouse and choice type (grey, recipient’s pokes, which are always directed towards the prosocial side; blue, focal’s pokes in the prosocial choice port; red, focal’s pokes in selfish port). Note that latency for focal choice varies depending on the trial, allowing the recipient to display its food-seeking behaviour or not. Circles indicate the moment where each animal visits the food-receptacle in their reward arm. Focal animals are always rewarded, and the colour of the filled circle indicates the type of trial after decision (blue, prosocial choice; red, selfish choice). Grey circles indicate time of receptacle visit for recipients, where filled circles correspond to prosocial trials, where recipient is also rewarded, and open circles to selfish trials, where no pellet is delivered.