Knock, knock. Who's there? Migrants. Millions upon millions of them, who would like to enter the United States in search of a better life.
No public opinion poll in the past 50 years has found a majority of Americans favoring increased immigration. Nonetheless, the total number of legal and illegal immigrants keeps growing, and the prospects of Congress acting to shrink the inflow are dim.
Is there a solution?
Over the long, long run, pressures to migrate to rich nations will ease as prosperity abroad spreads. Also, birth rates in poor nations - including Latin America - have been declining precipitously. Yet the world as a whole still adds 76 million people a year, mostly in the Far East and Africa, to today's 6.5 billion.
Since Mexico is the largest source of immigrants to the US, its population picture is the most relevant to citizens here. Each Mexican woman had an average of seven children in 1955, five children in 1980. Today, that number is 2.4. Within five years, as urbanization continues, Mexico's fertility rate should drop to 2.2. Soon thereafter, it could decline to 2.1, the "replacement rate," at which time Mexico's population will cease to grow - once the baby bulge has worked its way through the demographic situation.
By mid-century, though, Mexico will have about 139 million people, up from 107 million today. Unless there is a major improvement in the Mexican economy, the US will remain a big pot of gold to many Mexicans.
For decades, the supply of immigrants from many nations will be a large multiple of the US demand for new workers, notes Joseph Chamie, research director of the Center for Migration Studies, a New York think tank. The US must eventually put the brakes on illegal immigration, says Mr. Chamie, who was formerly head of the UN's Population Division.
If the status quo continues, the US population will soar from 300 million later this year to 400 million by 2050. The average age of Mexicans is 25, compared with 36 for Americans. So young Mexican immigrants on average will have far more children than American citizens will.
Without new immigrants, the US population will grow only to 320 million. But keeping the status quo in the US, including porous borders and not enforcing existing laws against hiring illegal aliens, is "politically advantageous," notes Chamie.
"It postpones making important - possibly painful - decisions, thereby avoiding alienating voters and important special-interest groups in an election year," he told the Population Association of America. Elected officials get "bulletproof" political cover by speaking of a "complex and difficult" issue, but not acting.
Despite the massive parades of mostly Hispanic immigrants in recent days; despite the congressional disputes over a host of immigration-related bills; despite the expressed compassion of many religious leaders for immigrant families, illegal and legal; despite the widespread use of euphemisms for the harsh-sounding but accurate word "illegal" ("undocumented," "overstayers," "irregular status") and political appeals for the Hispanic vote, the long-term impact of rapid population growth in the US gets short shrift in the national debate.
Some economists worry about what a continued inflow of immigrants will do to commuting times to work, urban sprawl, wear and tear on the environment (including national parks), the nation's religious balance, American culture, and the cost of public services.
One proposed remedy is a fence on the 1,800-mile border with Mexico. India has such a wall along parts of its border to the east to keep out poverty-stricken Bangladeshis and to the west to stop poorer Pakistanis, notes Chamie (who offered the "Knock, knock" riddle.)
A wall, however, would be expensive. Israel has been paying $2 million a mile for its wall blocking off the West Bank. At that rate, a Mexican wall would cost at least $3.6 billion, and probably more.
Steven Camarota, an economist at the Center for Immigration Studies in Washington, figures a fence combined with more border patrol agents could help reduce the inflow of illegal immigrants.
Further, he advocates "attrition" of the approximately 12 million illegals in the US - not mass deportation - through enforcement of existing laws. It took "25 year of neglect" to get to the immigrant situation of today, he says. It will take "a lot of years to get out of it."
Some 170,000 to 200,000 illegals leave voluntarily for home each year. A fence would make it harder for them to return to the US. Mr. Camarota suggests other obstacles: Make it difficult to get a driver's license, open a bank account, get public benefits, or use bogus Social Security numbers to get a job. Employers hiring illegal immigrants should receive serious fines, he says. State and local police should actively enforce existing immigration laws.
It may just be wishful thinking from a think tank advocating reduced immigration - though not an end to immigration.
Camarota's latest research finds that the surging inflow of low-skilled workers has been driving low-skilled native workers out of the labor force. They retire, live with relatives, or just drop out.
Between March 2000 and 2005, unemployment among less-educated adult citizens rose nearly 1 million. Another 1.5 million left the workforce altogether. In the same time frame, 1.6 million adult immigrants with high school or less education were added to the labor force. Immigration hits those at the bottom of the income ladder hardest.