Professor Daniel Wolpert explains why fighting children will often say that the other hit them harder:
You underestimate a force when you generate it, so as one child hits another, they predict the sensory movement consequences and subtract it off, thinking they’ve hit the other less hard than they have. Whereas the recipient doesn’t make the prediction so feels the full blow. So if they retaliate with the same force, it will appear to the first child to have been escalated.
Adults do it too:
This led to a simple but effective experiment being conducted called ‘tit for tat’, in which two adults sit opposite each other with their fingers on either side of a force transducer. They were asked to replicate the force demonstrated by each other when pushing against the others finger. Instead of remaining constant, a 70 percent escalation of force is recorded on each go. It seems that we really don’t know our own strength.
The Dish 