We've argued about the estimates for years. We've argued about whether illegal immigrant workers take jobs from US citizens (act as substitutes, in econ-speak) or do they allow US citizens to stay in better, perhaps supervisory, jobs (act as complements, in econ-speak). Now the Arizona legislature has decided that, arguments aside, these people must go. When I'm questioned about the impact of our "employer sanctions" legislation would be if it were successful, I have to ask what the goal of the law is. I usually get the response that the goal is to make illegal immigrants leave the state.

Given that goal and looking at the impact of its success is rather worrisome. The Pew Hispanic Center estimates that 8 percent of Arizona's population is here without the appropriate paperwork. If the law is successful in that given goal, one could expect Arizona to lose that portion of its population. But wait, there's more! Some of those people here illegally are related to, by marriage or birth, to US citizens. If we make the illegal immigrant leave, then surely we'll lose the US citizen too?

There is plenty of anecdotal evidence that this is happening. The problem is quantifying it. The US does not keep data on people that leave the country and a full census is only done once every ten years. For the population of illegal immigrants, driver license data is no help. School enrollment? Not a help either as many of the people who cross our border illegally leave their children at home and those that enter legally and overstay their visas may have children.

That leaves us in a quandry. Population growth helps drive economic growth, especially in Arizona. Take away that growth and we know the picture is not rosy. The problem is that we can't quantify how "not rosy" it could get.