I don’t pretend to even start to understand the laws of Mexico but I do know that citizenship is not automatic for children born in Mexico of foreign parents but it is possible or at least it definately was in the past. Many LDS including many of Romney’s family fled to Mexico to escape polygamy laws in the U.S. most became Mexican citizens, some have dual citizenship (as recognized by Mexico) and some are just citizens of the United States. Some families fled back to the United States during the Mexican Revolution to further cloud the issue of their citizenship. Some LDS are even moving back to the U.S. now from Mexico to flee the drug war so somehow at least some have managed to keep their U.S. citizenship as well as Mexican citizenship.
I have heard that at the time of the large LDS migration to Mexico children born in Mexico of foreign parents could choose their citizenship (not sure if that included everyone or was just a special rule for the LDS that moved there)- parents could choose Mexican citizenship for their children or the children could so do for themselves after they reached a certain age. I have been told by Mexican citizens that Romney could claim Mexican citizenship now if he chose to do so. As far as I have heard Mitt Romney has never showed any interest in his family’s Mexican heritage/history even though he has Aunts, Uncles, cousins, etc. that are very prominent, very wealthy Mexican citizens/residents.
Interesting - thanks!