Asians' passion for United States bonds is explained by their desire to stop their currencies appreciating against the dollar, and in doing so, they support the world economy, rather than undermine it. Toshihiko Fukui, Zhou Xiachuan, Joseph Yam, Perng Fai-nan or Park Seung, respectively bosses of the central banks of Japan, China, Hong Kong, Taiwan and South Korea, have become the world's most enthusiastic purchasers of American government debt. Between them, these central banks hold around $1.3 trillion in official reserves, most in dollar assets. China and Hong Kong fix their currencies against the dollar, in Hong Kong's case through a currency board. A current-account surplus or big capital inflows automatically translate into higher reserves. The other countries let their currencies float, but intervention by central banks has ensured that Japan's yen and South Korea's won rose over 13% against the dollar since the beginning of 2002, compared with a 25% increase for the euro, a floating currency. When Europe's finance ministers met their Asian counterparts at a summit in Bali in July, they made a fuss about weak currencies. America's Treasury secretary, John Snow, went to China earlier this month specifically to lobby for a change in the exchange-rate regime. And tensions are rising within Asia itself. The Japanese are unhappy about the undervaluation of China's currency, and the South Korean government suggested that China allow the yuan to appreciate. Rising foreign-exchange reserves are not necessarily a bad thing, and are particularly important for emerging economies. There is a suspicion that Fukui, Zhou, Fai-nan and company have been buying dollars for nefarious reasons: to keep their exports artificially cheap and hold on to their traditional export-led growth. The charter of the IMF prohibits a country from manipulating its currency to "gain an unfair competitive advantage" over its trading partners.