WASHINGTON—The European Parliament’s decision to pass a new law allowing member countries to imprison undocumented aliens for up to 18 months and deport children has reignited the debate over immigration, one of the most sensitive issues of our time.

The European arguments against immigration are similar to the ones that have been voiced in the United States, Canada and Australia, or, for that matter, in poorer countries that attract citizens from even poorer nations, such as Argentina, where many Bolivians live, and the Dominican Republic, which has a sizable Haitian population. European legislators and large numbers of Europe’s citizens believe that immigrants are taking jobs away from native citizens, endangering cultural values and undermining the welfare state.

Almost any society faced with a sudden influx of outsiders would feel understandably threatened. But the evidence flies in the face of all three of these fears.

La Caixa, a well-respected Spanish financial institution, measured the impact of immigration on Europe’s economy between 1995 and 2005. The result was staggering. For instance, the study found that immigration contributed on average 4.5 percent to the annual growth of Ireland’s economy and 3.8 percent to that of Spain. Had immigration not been a factor, Spain’s economy would have grown by just over 1 percent a year during that decade.

If we compare employment figures relating to the early 1990s, when there were fewer immigrants, with those of the new millennium, when most of Europe’s 8 million illegal aliens settled on the continent, it is obvious that immigrants don’t steal jobs. By expanding the economy with an additional supply of labor, immigration ultimately generates an even greater demand for workers. Which is exactly why Spain has much lower unemployment today—8.5 percent—than in the early ‘90s, when the rate was in double digits.

What about the cultural argument? Some immigrants bring in customs that make many natives cringe. Extreme cases involve mutilating a woman’s genitals or forcing a marriage between two children. More generally, some immigrants are not accustomed to the rule of law. Most people adapt themselves to new rules and customs because culture is ever changing. In any event, the threat to the host nation, in cases like the ones just mentioned, could only come from the inability of the authorities to enforce the law.

In some instances, the cultural case can also be made in the opposite direction. In various places, we have seen groups of native Europeans brutally attack immigrants—even those who are European. The most recent instance of violence occurred in Italy, where vigilante groups torched gypsy camps near Naples. Those thugs are not exactly the epitome of Western cultural refinement.

Finally, there is the welfare state. Ironically, it is immigrants who have given the kiss of life to some of Europe’s entitlement systems. In the 1980s and ‘90s, the continent’s low fertility rate meant that some European governments were not in a position to guarantee future pensions. Numerous countries started to toy with the idea of replacing their pay-as-you-go retirement schemes with Chilean-style private saving accounts. That idea has been laid to rest for now because the influx of migrant workers has expanded the base of contributors, pumping more tax money into retirement funds.

The eruption of violence in some of the ghettos that surround Paris is perhaps the best argument for reforming the welfare state that has anesthetized Europe’s economy in recent years. While the high tax burden and the stringent regulatory environment made it difficult to create businesses and hire new workers, the French welfare state gave many of those immigrants a free education, a free health service, unemployment benefits and the promise of an unearned pension. Giving them handouts but barring them from the possibility of work bred a dependency and resentment that led to their heinous acts of violence. The problem, then, was not immigration but the welfare state itself.

No society—not even one as open as that of the United States—is free from the fear of strangers. The modern arguments against immigration are a rationalization of that primeval instinct. It is unfair to expect European governments to act as if that instinct was not present in their societies. But passing laws to imprison immigrants and deport children is contrary to what we should expect from the continent that regards itself as the pinnacle of civilization.