Bogle's reasoning? First of all, he thinks U.S. businesses are and will remain the world's strongest thanks to their history of innovation, focus on technology, and foundations built on better legal and regulatory structures. As for other countries, he's wary of Britain's 'unusual electoral process,' Japan's demographics, and France's short workweek. But if you still want to invest abroad, Bogle says, don't go much above a 20% allocation—and certainly not to the 40% that many professional investors are recommending these days. That, he says, "is not worth it."