One-Fifth of an AmericanHow much is an immigrant's life worth, exactly? How do you justify a border fence? Why is it OK to consign millions of unskilled Mexicans to lives of desperate poverty? I'm told it's because Americans should care more about their countrymen than about a bunch of foreigners. OK, but how much more? Surely there's some limit; virtually nobody thinks, for example, that Americans should be allowed to hunt Mexicans for sport. So, exactly how much are you willing to hurt a foreigner to help an American? Is a foreigner's well-being worth three-quarters as much as an American's,...