Sovereign wealth funds have become a source of controversy. They have the size — several trillion dollars and growing — to swing or stabilize markets. Meanwhile, their sometimes secretive strategies have invited worries that they could be used as tools of government policy. Jeffrey E. Garten, former SOM dean and former undersecretary of commerce for international trade, talked to Ng Kok Song, the managing director and group chief investment officer at the Government of Singapore Investment Corporation, about how one of the world’s largest SWFs is run.