Prism - it is a social and moral norm that is universally understood that able bodied people should give up their seat for ones who are not so fortunate at that given time. Examples - disabled, pregnant, elderly. It is not a question of people behaving as I should want, it is what is normal in any caring society. If i were not pregnant or any of the above, I agree I should not expect anyone to give up their seat to me.

TB - agreed, but sadly it was pulled a while back