Mexicans have a lot of blood and genetic heritage in common with Native Americans (Indians). Indians were only recently introduced to alcohol and have significantly less tolerance for it then Europeans who have been imbibing alcoholic beverages for millenniums. Alcoholism is a tragic plague on the reservations around Wyoming and Montana that I am familiar with. Mexican Indians might be slightly more tolerant because they did have some fermented cactus drinks before the Spanish got there. I think they might have called it "pulque". It was essentially tequila that had not been concentrated through a distillation process.
In any case, I would not be at all surprised to learn that Mexican were genetically predisposed towards alcoholism. I would also not be surprised to learn that there was a direct correlation between darker skin pigment (more Indian blood and less European blood) and predisposition towards alcoholism.
You said -- "Mexicans have always had a drinking problem.. We used to live in So. Calif., and every Sunday morning you would see them lined up with their pats and pans for menudo to combat a hangover."
I've seen a few of them driving around in Dallas (where I happen to be at the moment) and they're drinking with a bottle kept between their legs. They don't seem to mind "drinking and driving" apparently. I don't know if this is an overall trend with them in general -- but -- I've already noticed it on my own, just around here.
And I've seen a number of them come staggering (and I mean "literally staggering") out of bars and jumping into cars (or pickups, which seem to be a favorite around here...) and driving off. It makes me worried about driving around here at times.
Regards,
Star Traveler