KEEPING unwanted immigrants out of America is a costly business. Thousands of miles of fence have been erected along the border with Mexico and the annual budget for the US Border Patrol has risen to $2.7 billion in 2009 from $326.2m in 1992. Nonetheless, illegal immigration is reckoned to have trebled in that time, in part because many people enter the country legally, for example as tourists, but then stay on permanently. According to new data from the Center for American Progress, a think-tank, the cost of apprehending immigrants is rising. Over 791,000 arrests were made by the border patrol in 2008 (the latest year for which data are available), at a cost of just over $3,000 for each arrest.

AP