AMERICANS never made up a large portion of Swiss private banks’ international client base, but the price to be paid for allowing some of them to evade tax is proving to be steep—and could be ruinous for some smaller wealth managers. By December 9th most of Switzerland’s 300 or so banks were required to tell their regulator whether they would participate in a voluntary-disclosure programme crafted by the Department of Justice, under which those that have handled untaxed accounts for American clients can wipe the slate clean in exchange for fines. Swiss authorities have urged banks to sign up to avoid the fate of Wegelin, a venerable private bank that closed after being indicted in New York for aiding tax dodgers.Banks have four choices. They can declare themselves “category two” institutions (those with foreign clients who broke American tax laws), “category three” (those whose foreign clients were clean), “category four” (mostly domestic) or they can choose not to take part. “Category one” comprises 14 large banks, including Credit Suisse and Julius Bär, which cannot participate because they were already under investigation when the programme was set up. They will have to negotiate individual settlements with the American authorities. The only large bank that has already done so is UBS, which paid $780m and handed over information on more than 4,700 American accounts….