The empathy-altruism hypothesis postulates that the awareness of others’ need, pain, or distress increases empathetic feelings, which in turn triggers cooperative behaviour. Although some evidence supports this hypothesis, previous studies were prone to the ‘experimenter demand effects’ raising concerns about the interpretation of the results. To avoid this issue, we designed a laboratory experiment where we examined whether the presence of individuals with a genuine physical disability would increase group cooperation in a public goods game. By manipulating the group composition during a social dilemma, we created a more ecologically valid environment closer to real-life interactions. Our results showed that the presence of physically disabled individuals did not affect group cooperation. Specifically, their presence did not affect the contributions of their physically abled partners. The lack of a surge in cooperative behaviour questions the interpretation of previous studies and suggests that they may be explained by an experimenter demand effect. Alternatively, our results may also suggest that in the context of a social dilemma with real stakes, people with physical disabilities are not perceived as being in need or do not induce enough empathy to overweight the cost of cooperation and trigger cooperative behaviours.