Researchers tested the theory by asking subjects to judge the weight of a box lifted by an actor while the subject lifted a light (150g) or heavy (750g) box. The scientists found that actively lifting a box altered the subject's judgment of the weight of the actor's box. The box that was being lifted by the actor was judged to be heavier than it really was when the subjects were themselves lifting a light box. But when the subjects were lifting a heavy box, they thought the actor's box was lighter than it really was.
The results are compatible with a current cognitive neuroscience theory known as 'Simulation theory'. According to this theory, we understand other people's actions by imagining doing the same thing ourselves. This means that we use our motor system in the brain to perform perceptual tasks like observing other people. Our motor system can accurately estimate the difficulty of another person's task if we are just sitting still and watching, but if we are doing something else at the same time, like lifting a box, then some of the motor processes are busy and can't be used to judge what the other person is doing, making us less objective.
Dr Hamilton said: "Our brains are wired to see other people's actions in relation to our own. Mixing up our actions with other people's actions is part of what makes us very good in social situations, but this new research has shown that this ability also biases our judgments, making it seem that someone else always has the easier job!"