If you ever wonder why Banamex, the No. 2 bank in Mexico, is so key to the fate of parent company Citigroup, I can explain.

In recent years, Banamex has been the source of anywhere between 11 and 15 percent of the global profits of Citigroup. Why? Because Banamex nicks clients with fees far higher than the global average.

My wife and I recently miscalculated and I wrote a check with insufficient funds on our Banamex account. The bank has been unable to provide me with a monthly statement since last August but they very quickly sent a note about the charges I had incurred for the overdraft: 642.72 pesos. That is more than $50.

According to this website, overdraft fees in the United States range from $10 to $38, with a median fee of $27.

Of course, Banamex is not alone in charging Mexicans far more than prevailing global rates. Look at cellular phone and cable TV rates, airfares from Aeromexico, or any of a range of services offered by companies with a strong market share. In Mexico’s sclerotic political system, politicians gain more by favoring powerful companies than average people.