We use an extremely simple setup with minimal strategic concerns or potential for misunderstanding to identify experimenter demand effects against alternative expla nations. By providing an equivalent physical task to all subjects regardless of whether coupons are destroyed, we control for the pleasure of physical activity. Our experiment also controls for the potential benefits of coupon destruction for the coupons provider and for the clarity of the instructions. We are then able to separate out experimenter demand effects from altruism towards the experimenter as possible explanations of behaviour. If subjects are driven by altruism towards the experimenter in deciding what to do in experiments with variable surplus tasks, we would expect subjects to return money to the experimenter but not to destroy the coupons as this could not ben- efit the experimenter. demand effects drive both physical Conversely, if experimenter coupon destruction and returning money to the experimenter, both should be positive and should be positively correlated to each other