* Diverse asset classes all rising simultaneously
* Liquidity response by central banks seen as key
* Dollar down as other currencies benefit
By Nick Olivari
NEW YORK, Sept 11 (Reuters) - General investment theory holds that investors should diversify, buying one asset class on expectations of profit as they avoid others on fears of a loss for any given set of economic circumstances.
But with gold at 18-month highs, U.S. stocks rising off recessionary lows and a rally in U.S. bonds sending yields on some debt to the lowest since July, the investment climate is anything but typical.
The worst global economic contraction in decades prompted low interest rates around the world and central banks, including the U.S. Federal Reserve, to take the extraordinary precaution of buying the debt of their own government in a bid to keep liquidity in the system.
But as economies recover, that liquidity has to find a home.
The big question is how long the atypical investment climate will remain. Much of that depends on when central banks end those extraordinary actions, known as quantitative easing (QE), and why they end it.
"Asset classes are up due to massive credit, fiscal, and monetary stimulation that is bleeding into them," said Andrew Busch, global foreign exchange strategist at BMO Capital Markets in Chicago.
Spot gold
"Essentially, this bullish action in several markets may be a sign that the 109 percent year-over-year increase in the monetary base may finally start translating into money supply growth," said Mike O'Rourke, chief market strategist at BTIG, in a note to clients.
"As usual, the improvements appear in the financial markets first, and the real economy later."
The dramatic run-up in bond prices has occurred despite the dollar deteriorating, data showing further economic stabilization and Wall Street hovering at a one-year high.
Bond bulls say the economy remains vulnerable with consumers restraining their spending as they try to heal household balance sheets battered by fallen home values and shrunken stock portfolios.
Gold, meanwhile, has benefited from a decline in the U.S. dollar with recovery hopes fueling interest in currencies seen as higher risk.
Those same hopes are helping to send stock markets higher on expectations that economic recovery will boost corporate profits.
Ending QE because of a return to worldwide economic growth would be an acknowledgment that the crisis is past. Markets should begin to function normally with investors buying one asset class at the expense of another while the Fed begins to mop up liquidity. Improved economic demand would likely benefit stocks, to the detriment of the bond market.
Alternatively, if QE is left too long and the global investment climate becomes inflationary, financial markets could be in for another period of turmoil -- one that could negatively impact bonds and stocks.
"The danger is that if growth does return, then inflation will cut it off at the knees due to high prices," said BMO Capital Markets' Busch. (Reporting by Nick Olivari; Editing by Kenneth Barry)