Sovereign investors tend to be content with holding stakes over the long term and don't mind waiting for a decent return. But those who invested as problems first began to surface in 2007 and early 2008 can't help but look back and realize their timing was way off. Others who stepped away from investments in now-defunct firms Bear Stearns and Lehman Brothers can't help but look back with a sigh of relief. As a result, some experts suggest that sovereign wealth may not flow so freely the next time U.S. banks are in a bind. "The real concern there has been in terms of their return on investment," says Michel Léonard, chief economist and macro-strategist at Alliant Insurance, a Blackstone subsidiary. "If I were concerned about these funds investing again, I would look at some of the high-profile investments in U.S. banks, for example, which were highly costly to those funds." Léonard notes that some sovereign wealth funds have "very publicly" expressed disappointment that U.S. investments that didn't turn out to be "as attractive as they had wanted." Still, their crisis-era direct investments are subject to lock-up periods that have costly provisions if investors want to pull money out sooner. As a result, Léonard doesn't think sovereign wealth funds will pull their money out early - he just thinks they may not come back if and when big banks need big bucks to cover losses, expand their businesses or engage in expensive M&A. Sovereign wealth funds have been a great source of capital for the global marketplace because of their size and scope. Estimates vary, but sovereign wealth funds are thought to control anywhere from $2.5 trillion to $4 trillion in assets, according to Herb Kaufman, a former Fannie Mae ( FNMA,OB) economist and consultant to the U.S. Treasury, the World Bank and the Congressional Budget Office, who is professor emeritus at the W. P. Carey School of Business at Arizona State University. The Middle East and North Africa are thought to control about $1 trillion worth of those funds, thanks largely to the wealth of petroleum resources in the region.