上周，华尔街牛市迈过了多个重要的里程碑：标准普尔500指数的收盘点位达到了2009年熊市低点的两倍；罗素2000小型股指数收盘创下历史新高，收复了2008-2009年熊市的所有失地；以科技股为主的纳斯达克综合指数也涨至十年来的高点。

    人们很容易将这轮反弹归功于一系列令人鼓舞的经济数据。但真正的原因可能更多是因为投资者对于现金投资几乎零回报的不满情绪。

    没错，上周三美国股市出现反弹之时，恰逢有报道称，欧洲央行（European Central Bank）为了避免让负债累累的欧洲国家陷入信贷紧缩，将为这些国家的银行提供超低利率贷款。与此同时，美国商务部（Commerce Department）也宣布，美国经济增速快于2011年底时的最初预期。而且，美国劳工部（U.S. Labor Department）上上周公布，美国申请失业救济人数的四周均值已降至四年来的最低。

    这些因素可能是推动投资者加速进入股市的润滑剂。但此轮股市上涨之时，也正是投资者苦于没有好的投资渠道之时。正如韦斯特伍德控股集团（Westwood Holdings Group）的首席投资官马克•弗里曼所说，借贷成本已降至前所未有的低位。事实上，追逐安全投资意味着如今投资者借钱给政府还得倒贴利息。

    过去五年，美国政府支付的平均债券利率逐年下降，已从2006年底的4.92%降至2011年底的2.24%。今年以来，利率进一步下降。弗里曼说，由于通胀率在2%左右，现金的实际回报率已处于负值。

    他对这波股市反弹行情的看法是：“发生变化的是信心在某种程度上的提升，”他说。“投资者现在都说，‘我的现金回报率还是负值，但情况似乎有一点点好转了。因此，我可能会投资一些风险较高的产品。’”

    美联储（Federal Reserve）在启动之前的两轮国债购买行动时，大致也是这么想的——最近一次是去年9月的“扭曲操作”，通过推低长期利率，带来低成本的宽松资金。韦斯特伍德2月份的一份研究报告指出，随着美国大公司手头的现金堆积，派息再次开始增加。标普500家的总派息已从2009年的1,960亿美元增至2011年的2,410亿美元。2012年，预计派息将达到创纪录的2,750亿美元。

    诚然，美联储的宽松货币政策就算谈不上稳定股市，也至少帮助支撑了市场。但未来股市要继续反弹，难度会更大。迄今为止，还没有任何迹象显示美联储会启动新一轮国债购买行动。蛋糕缺少了糖霜、只有基础配料（也就是美国经济的基本面转强），看上去光秃秃的。

    Wall Street's bull market reached a few milestones this week: The S&P 500 index closed at double its bear market low from 2009. The Russell 2000 small-stock index closed at a new all-time high, erasing all of its 2008-2009 bear-market losses. And the tech-dominated Nasdaq composite index rose to its highest point in a decade.

    It's easy to attribute the rally to the slew of cheery economic data. But what's really driving the bulls has more to do with investors frustrated with virtually zero returns on cash.

    True, the market rallied Wednesday amid news that the European Central Bank – in efforts to keep debt-troubled Europe from slipping into a credit crunch – would offer banks ultra-low interest loans. Stocks also rose as the Commerce Department reported that the U.S. economy grew faster than originally thought at the end of 2011. What's more, the U.S. Labor Department reported last week that the four-week average of applications for Americans seeking unemployment aid fell to its lowest point in four years.

    Such factors might be the grease on the wheels driving investors into stocks. However, the gains also come as investors find themselves with few places to get decent returns. As Mark Freeman, chief investment officer of Westwood Holdings Group (WHG), points out, the cost of borrowing is as low as it has ever been. Effectively, the flight to safety means investors are paying to lend money to the government.

    For each of the past five years, the average rates that the government pays on in its debt have declined -- from 4.92% at the end of 2006 to 2.24% at the end of 2011. Rates have fallen further so far this year. With inflation running around 2%, that puts real returns on cash in negative territory, Freeman says.

    He sees the stock rally this way: "What has changed at the margin is that confidence has improved some," he says. "Investors are now saying, 'Hey, I am still getting negative returns on cash and things seem to be doing a little bit better. So maybe I'll invest in something riskier."

    And that's generally what the Federal Reserve had in mind when it launched two rounds of bond purchases – the latest in September under what's called "Operation Twist," which promised cheap and easy money by driving down long-term interest rates. In a February research note, Westwood noted that dividends are beginning to grow again as cash at the biggest corporations piles up. Total dividends paid out by S&P 500 companies rose from $196 billion in 2009 to $241 billion in 2011. In 2012, dividends are expected to be a record $275 billion.

    Indeed, the Fed's easy money policy has helped prop up, if not stabilize, the market thus far. But it's going to be tougher for the stock market rally to continue. So far, the Fed has not signaled any signs it will launch another round of bond purchases. Without the sweet icing – if you will – the cake is left bare with only its basic ingredients. That is, the fundamental strengths of the economy.