social behavior in the ladies' room

There is a strange ladies' room dynamic at bars and restaurants that have only a single stall. When there is a line, the person leaving the bathroom invariably half-smiles at the first person in line, and the smile is returned. I know this doesn't happen with the men's room, but I've seen it every single time I've been in this situation. I do it too. I can't help it, and I never thought about it until last night, when it happened yet again.

The leaver smiles as if to say "sorry I took so long."

The enterer smiles back to say "that's ok." But she really means, "bitch I have to pee so damn bad. Just wait til I'm finished, then I will kill you."

The leaver's smile translates to "I'm not sorry. You should have gotten here earlier."

I know it makes women sound catty and bitchy, but it's true. Ladies, don't try to deny it. It may be entirely subconscious - ingrained in the female brain after generations of social niceties - but you do it. I do it. Every woman does it. I have a feeling that if I pulled an aborigine out of the bush and put her in line for a small public restroom, that she would then give the false half smile.
There is an unspoken rule about it.

Quick - someone call a sociologist.