The recent debate over the Arizona immigration laws and immigration law in general has left me reflecting on why governments insist on passing laws, and people continue to demand them, that will be impossible to enforce. There is a lot of data showing that it is so prohibitively expensive to reduce supply of just about anything by enforcement measures alone that I am left at a loss to give a rational reason (I am not however at a loss to give plenty of irrational reasons) why we continue to pretend that we can. We know we can force supply into controlled channels and we can create incentives for supply to favor these channels over breaking the law but while these methods are sufficient to have marginal effects on both supply and demand there is no mechanism by which we can force either to conform to an arbitrary standard of what we'd like immigration or anything else to be at. If you restrict something like immigration too much, illegal means will be found of meeting demand. It has always been so, whether you're talking about tea smuggling in the American colonies, drug supply or illegal immigration today, or any other of an near infinite number of policies imposed that were meant to restrict economic or social life into some sort of preconceived pattern rather than trying to control the pattern that already existed.
Yet, whenever laws fail to make people behave how we'd like them to the response is to call for greater use of force to make them conform to our standards. However, unless expenditure is on a truly massive scale, this never works. One of the more absurd examples I've read about recently is in Braudel. To choose a frivolous non-immigration example (the inability for lords to prevent peasants from running to cities and the large population migrations show it was far from possible to control this then and you can go back to Rome for other examples of uncontrolled immigration) that shows the futility of using punishment to control supply and demand I will tell the tale of France and Indian cotton.
Against this invasion of light fabrics nothing worked. Not "supervision, inspections, confiscation, imprisonment, fines..." or advisers who suggested "to strip... in the street, any woman wearing Indian fabrics." (179) Prosecution failed to stop this, even with confiscation of goods and fines of over 1000 ecus (don't know the conversion) on anyone who would buy and sell the fabric. So devastating was this scourge that in 1717 punishments were increased to deter these illegal fashionable by making punishments harsher, including among others a sentence of life in the galleys, "and even worse if the case called for it." (180) This invasion of light fabrics and the illegal fashionable would not cease until France lifted the ban in 1759 and gained a cotton industry able to compete with others.
The point of this? If life in a galley wouldn't deter people from wearing cotton, the Arizona law, and any immigration enforcement we try, isn't going to deter people from seeking a better life here. Supply and demand simply doesn't allow these kinds of laws to function, you can't set supply or demand by fiat without some really heavy guns to force it to work. Best accept reality as it is and develop policies to control immigration and prevent the illegal and socially corrosive activities that come with illegal immigration by creating legal channels for it.