The first was a demographic bulge caused by Mexico's high birth rate: In 1960, the average Mexican woman had seven children. Today, the figure is down to nearly two. Mexico no longer produces enough people to export them in the volume it once did.
Another difference is the difficulty of getting here. "Border security is vastly more robust than before," Alden told me. Over the past two decades, he says, the number of Border Patrol agents has soared from 3,000 to 21,000, while 700 miles of fencing has been erected on our southern border.
Back then, walking in undetected was hard only if you had trouble walking. Today, the chances of arrest are high, and many aspiring entrants pay thousands of dollars to smugglers to improve their odds.
Fewer people take the risk these days. Despite having far more personnel, the Border Patrol apprehended fewer people in 2011 than in any year since 1972.
Not everyone wades the Rio Grande. Lots of the undocumented came here legally as students or tourists and overstayed their visas. But that tunnel has also gotten narrow.
The 9/11 attacks brought stricter checks and new controls on many foreigners coming here legally. In 2011, nearly two million visa applicants were rejected. That's nearly two million foreigners who couldn't become illegal immigrants.
Any comprehensive immigration measure won't ease those requirements. In fact, the senators insist on new steps to assure that those who come from abroad legally actually leave when their time is up. Nor will a reform bill turn the Border Patrol into the Welcome Wagon.
But the critics are right: If illegal immigrants gain a path to legal status and citizenship, the change is likely to evoke "Groundhog Day." In the movie, every day begins the same way. What changes is what happens next. In the end, remember, they live happily ever after.