According to espndeportes.com, there have been 92 red cards in the first 160 games of the current Mexican season (1.7 games per expulsion). In the 150 games of the current MLS season, there were a grand total of 37 red cards (4.1 gpe). It's more work to extract the total number of yellow cards in Mexico, but there appears to be a large disparity in that category, too ... conservatively, at least 50% more down south. Frankly, I don't think the games are dirtier or out-of-control down there; I just think the refs are different. In general, MLS has some of the most lenient refereeing I've seen. This is something to remember when we're wondering why so many US players get sent off in matches abroad. (That said, I do suspect that our teams get screwed over a lot, too.)
I think an interesting contrast, beineke, might be division I in England. Mark Williams, in his short stay here, was the most egregious fouler in the league (a yellow card a game, 2 PK's), and complained repeatedly about how strict MLS referees were. I don't doubt that our refs are much more lax than MFL's and so it seems pretty unlikely that Division I could be that much more lax.