I've always thought that airlines are more likely to upgrade their own nationals - purely subjective on my part, but I've always had more upgrades on outbound legs than returns. It's probably down to a certain "easiness" with staff at checkin, rather than any inate nationalism/favouritism.
Yeah that's always been my experience, particularly with BA. As a non-Brit UK resident that gets my attention! However, a noble exception appears to be United - they seem to be astonishingly even-handed, on occasion giving the upgrade to me whilst refusing one of thir own. That also gets my attention, and ensures that they get my business
