On U.S. "economic exceptionalism," Wien noted the relative strength of American economy following the global recession of 2008. U.S. stocks, the dollar and gross domestic product have all outperformed other developed countries.
"While the United States has many social and government policy problems that are likely to be with us for some time, there is little question that the performance of the economy since the Great Recession has been impressive compared to other industrialized countries," he wrote.
Wien noted that such a position doesn't guarantee endless strong stock returns, but said he still doesn't believe valuations of companies are yet excessive.
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On oil, Wien predicted price levels would rise as demand, particularly from emerging markets like China, India and the Middle East, picks up. He also said U.S. shale production, a key reason for the surge in supply, will remain modest until prices increase.
"Taking a look at past periods when the price of oil has had sharp declines shows a consistent pattern of rapid, not slow, recoveries," Wien said, noting he expects a price increase by year end.
On Europe, Wien said he had changed his mind from a few months ago on the region's economic recovery.
He noted that the Ukraine-Russia ceasefire and a resolution to the Greek debt crisis have added stability. Combined with the European Central Bank stimulus program, a weak currency—good for exports—and the low price of oil, the euro zone will not be stuck in a recession and deflation.
Finally, Wien voiced optimism on Japan. He noted strong corporate profits, wage growth and corporate share buybacks as examples that the economy was better off than many investors believe.
Plus, Wien said, officials are committed to more stimulus if necessary.
"If the increase in inflation and growth does not happen because of natural economic forces, the Bank of Japan will increase the money supply to make it more likely," Wien wrote. "Either way the outlook for the Japanese economy and its stock market is favorable."
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The Blackstone Group has almost $300 billion in assets under management, including private equity, real estate and credit funds. The firm is led by co-founder and CEO Stephen Schwarzman.