I have specifically had this explained to me as being due to the bystander effect. "Someone call 999/911" - chances are no one will do it, due to the bystander

I have specifically had this explained to me as being due to the bystander effect. "Someone call 999/911" - chances are no one will do it, due to the bystander effect; "You (points) call 999/911" - they probably will.