Wall Street's biggest banks say the slump in emerging-market assets that left equities trailing advanced-nation shares by the most since 1998 last year will prove more than a fleeting selloff.

Goldman Sachs Group Inc. recommends investors cut allocations in developing nations by a third, forecasting "significant underperformance" for stocks, bonds, and currencies over the next 10 years. JPMorgan Chase & Co. expects local-currency bonds to post 10 percent of their average returns since 2004 in the coming year, while Morgan Stanley projects the Brazilian real, Turkish lira, and Russian ruble will extend declines after tumbling as much as 17 percent in 2013.

While the economies of Brazil, Russia, India, and China symbolized the increasing power of the developing world during the worst of the global financial crisis and delivered outsized returns, Morgan Stanley says some of the same nations may now prove to be laggards as the U.S. Federal Reserve scales back unprecedented stimulus and interest rates rise. The MSCI Emerging Markets Index is down 3 percent this year, compared with a 1.1 percent drop in the developed-market index, and hit a four-month low yesterday as data from China showed weakness in manufacturing and services.