The big immigration debate is often the big obscurantism debate. The wool is pulled over our eyes and obtaining clarity is not easy.
The vested interests in continued immigration are enormous - first and foremost, the migrants themselves who are seeking an escape from poverty and lack of opportunity at home.
The great migration of Mexicans to the US is now a trickle - mainly because of fast-growing work and educational opportunities at home
They are supported by governments at home who look at the remittances that bail out their balance of payments problems without looking at the other side of the balance sheet - the loss of the "best and brightest" from their own economies, the often sad and destructive impact on family life, and migrants spending, when they do come home, on a house and consumer items rather than on investment either in their farm or a small business.
Likewise, governments and employers in receiving countries tend to look at immigration in a lopsided way. For them, it is a short cut - to keeping wages down, to filling the jobs that locals won't do, to working night shifts, dangerous jobs on building sites or filling the shortfalls in seasonal labour on farms.
It is the easy way out. Job retraining on a massive scale, persuading companies to initiate work methods that attract native workers, and raising the retirement age so that older members of society can continue to contribute, are more daunting.
Receiving countries have long taken a simplistic view of the long-term costs of immigration. First-generation immigrants are young and vigorous, prepared to work long hours at unpleasant jobs, pay taxes and draw less on social funds and health services. Their crime and unemployment rates are low, and they dream of retiring back home.
But governments have ignored the costs - in particular, the attitude of the sons and daughters of immigrants. After poor education, they have adopted the attitudes of their local working class peers. They are not going to do the base work their parents did. A good number turn to crime.
Governments faced with uncompetitive industries have heaved a sigh of relief that immigrants can keep the show on the road. In Britain, in the 1960s, it was easier to allow the cotton and wool mills of northern England to import workers from Pakistani villagers to work long hours and night shifts than allow the industry to go bust.
But go bust it eventually did, in the face of overwhelming competition from Third World producers and a reduction in trade barriers. The legacy is a bitter second generation who feel betrayed by their parents and the government. No wonder they are ripe for picking by Islamist militants.
Did Western governments think for two minutes what the build-up of large flows of migrants was having on their own population? Rarely. Right-of-centre politicians thought of the economic benefits. Left-of-centre politicians thought of the value of diversity.
But the local working class who rub shoulders with the immigrants became tired of taking the brunt of policies imposed by the elites who don't. Hence, the rise of right-wing political parties in Europe and unsettled feelings in the Gulf states.
But there is some good news. According to a long investigation by The New York Times, the great migration of Mexicans to the US is now a trickle - mainly because of fast-growing work and educational opportunities at home, a sharp fall in the birth rate and a desire to avoid the drug and people traffickers on the border.
Twenty years ago, the same thing happened with Puerto Rico. There's the answer - economic development in the sending countries and retraining, increasing motivation and upping the retirement age in the receiving countries.
Jonathan Power is a syndicated columnist