The fierce debate on immigration ignores a crucial reality -- what is happening to the United States is only one piece, although a big one, of a much larger global picture.

That hit me a couple of weeks ago when I was in Singapore. The Southeast Asian island nation has long been hailed as an economic model, the business capital for the entire region.

But it is an economy facing demographic peril. Its small population of 4 million is shrinking, thanks to a very low fertility rate. Prosperous Singaporean couples work hard, have fewer children and worry about how to take care of their aging parents. By 2050, Singapore will have a median age of over 52, one of the oldest in the world.

Singapore's answer is to import labor. A third of its workforce are migrants, from construction workers to maids. One out of seven households employs a domestic worker -- low-paid women mostly from neighboring Philippines and Indonesia.

Singapore tries to lure "talents'' -- highly skilled and affluent migrants -- to stay permanently. But the men hauling bricks and the maids washing laundry are in a separate class of temporary guest workers, with no chance to join Singaporean society. If a maid becomes pregnant, she is shipped out within seven days. Employers have to post bonds that must be paid should their servants break the rules and try to stay, putting them in the role of migrant police.

Problems of abuse of domestic workers, including physical and sexual violence and confinement, are serious enough to have prompted a report last December by Human Rights Watch.

Singapore's dependence on migrant labor and its guest-worker policy may be at the extreme end but it's very much on the global spectrum. Labor, like capital and goods before it, is part of a global market. The movement of people across borders in search of wages and work, most of it from developing countries to developed, is growing at a phenomenal pace.

The numbers are staggering. From 1980 to 2000, the number of migrants living in the developed world more than doubled from 48 million to 110 million. Migrants make up an average 12 percent of the workforce in high-income countries. About 4 million migrants cross borders illegally every year.

The demand for labor is driven in part by a demographic disaster -- the falling birth rates of developed countries. Almost all of those countries now have fertility rates that are well below 2.1, the level at which a population replaces itself. At the very low end are Hong Kong (0.94), Korea (1.22) and Singapore in Asia (1.24), along with much of Eastern Europe.

Low fertility means shrinking workforces and aging populations. Without migration, according to a recent study, Europe's population would have declined by 4.4 million from 1995 to 2000. Immigration accounted for 75 percent of U.S. population growth during the same period.

This movement of people cannot be stopped, certainly not by hundreds of miles of fences or even by tens of thousands of border guards. It is an issue that cries out for global cooperation, for common policies that cut across national boundaries. Already, we can benefit from looking at what has worked -- and not worked -- elsewhere.

A Global Commission on International Migration, formed in 2003 by the United Nations secretary-general, has taken an initial stab. Their report, issued last winter, supports the growth of guest-worker programs.

The Senate immigration bill now up for debate includes a provision for a guest-worker program. The bill is clearly preferable to the punitive and ineffective approach of the House version. But the Singapore experience -- and previous guest-worker programs like the German import of Turks -- should prompt second thoughts about going down this road.

One problem is that the guests don't leave. The United States has its own experience with this in the bracero program to import farmworkers, and more recently with the supposedly temporary H1-B visas used so extensively by the high-tech industry here in Silicon Valley.

Most troubling to me, these programs create an underclass of migrants who are never assimilated, as happened in Germany. It sets us on the Singapore road, encouraging inhumane policing mechanisms. And it is a gilded invitation to employers to depress the wages and incomes of American workers, and not just in the dirty jobs that are supposedly so hard to fill.

The United States has been rightfully proud of a tradition that treats all immigrants as citizens in the making. Rather than guest workers, isn't it more American to set realistic immigration quotas and enforce them fairly?

Divorce proceedings bring out the worst in people. When Abdul Rahman tried to get custody of his daughters in Kabul, Afghanistan, his wife's family told the court that he was unfit to care for his children because he had converted from Islam to Christianity some 16 years ago. A zealous prosecutor, hearing of the case, charged Mr. Rahman with apostasy, a crime punishable by death under some interpretations of Islamic law. If Mr. Rahman does not repudiate Christianity, the judge in the case has said, he will get the death penalty.

Mr. Rahman's case is a discouraging illustration of the uneasy balance between the democratic norms Afghanistan's Constitution enshrines and the conservative Islamic values its judiciary upholds. On the one hand, the Afghan Constitution states that "followers of other religions are free to exercise their faith and perform their religious rites within the limits of the provisions of the law," and it requires the state to adhere to the Universal Declaration of Human Rights, which clearly protects freedom of conscience and the right to change one's religion.

On the other hand, the Constitution also says that no law can be "contrary to the beliefs and provisions of Islam," and it gives judges broad power to interpret and apply Islamic law. Several schools of Islam do indeed prescribe the ultimate punishment for those who abandon the faith. And so Mr. Rahman's case may well come down to the interpretive leanings of the court.

Moderate Islamic jurists in some countries have attempted to balance or reconcile these often-conflicting interests. In Egypt, for instance, the Islamic Research Center decreed that although apostasy may be a crime, the time period for redemption is limitless - in other words, it is up to the individual, not the state, to adhere to divine will. The former chief justice of Pakistan, which has explicit anti-blasphemy laws, has written that the death penalty for apostasy is not required by the Koran and conflicts with other Islamic values.

Afghanistan's post-Taliban judiciary, however, has shown a propensity to use Islam as a political weapon. The country's chief justice, Fazil Hadi Shinwari, is a hard-line conservative associated with the Islamist parties of Abdul Rasul Sayyaf and Burhanuddin Rabbani. He has used the court as a bully-pulpit, issuing fatwas on a variety of issues outside his jurisdiction.

For instance, under Justice Shinwari's leadership the Supreme Court has variously attempted to ban co-education; tried to eliminate a rival to President Hamid Karzai from the 2004 elections; and jailed newspaper editors, all in the name of Islam.

In other words, the court has overstepped its bounds and contributed to the radicalization of Afghan politics in the process. To further his aims, Justice Shinwari has packed the lower courts with judges who have Islamic educations but no foundation in Afghan law or experience in the judiciary.

President Karzai has a unique opportunity to change this. Under the Constitution, Mr. Karzai must appoint a new Supreme Court this month, and he sent his slate of nine justices to Parliament for approval last week. Although the current chief justice has retained his position, there are some very promising choices among the eight other justices. They include known moderates, like the former chairman of the Judicial Reform Commission, Bahauddin Baha, and the deputy minister of justice, Qasim Hashimzai, who led a major corruption investigation involving members of President Karzai's cabinet.

These appointments mark President Karzai's first opportunity to compose Afghanistan's Supreme Court under a fully constitutional government. They are of momentous importance to the country's stabilization and the consolidation of its nascent democracy.

By creating a competent, professional and moderate judiciary, President Karzai will help to establish the rule of law. If, however, the court remains in the thrall of ideology and factionalism, Afghanistan's experiment in democracy will be compromised.

But the new judges will be powerless to reform the system unless they are given the political support and resources to do so. International involvement in Afghanistan's justice sector since 2001 has been inadequate. Both the Afghan government and its donors need a strategic vision for the judiciary's future and the political focus to make it a reality.

The new judiciary will need support to review the qualifications of the lower court judges, facilities to train new judges and functioning courthouses in the provinces. It will need to be able to share information, laws and legal decisions among officials throughout the country and to pay judges a living wage.

We must do more than simply react loudly to the most extreme cases, like that of Mr. Rahman. Instead, we must partner with the Afghans and other democratic governments in the Islamic world as they struggle to promote modernity and the rule of law. This means working with judicial systems on less controversial, bread-and-butter issues like criminal law and property disputes.

We have seen throughout the world, and in our own history, that competent and independent judges will stand up for the rule of law even when their decisions indict the powerful and defend the unpopular. Mr. Rahman's case should remind us of how important it is to help Afghanistan develop such judges if we want its democracy to succeed.