Shareholders in acquiring companies should be thankful: mergers and acquisitions are down sharply this year.

Despite oceans of happy talk after nearly every merger about new synergies, efficiencies and growth, the actual outcomes facing owners of the bidding companies are far less rosy.

Performance among Russell 3000 stocks which closed a merger between 2001 and 2013 lagged the broader market by more than 16 percentage points in the three years after the deal, according to an August study by S&P Global Market Intelligence. And if you think that's merely a reflection of companies in hard-hit sectors merging to manage decline, buyers also underperformed their industry by 12 percentage points over the same period.

After deals close, profit margins, earnings growth and return on capital all fall, due in substantial part to heavier debt loads, interest expense and the tendency of buyers to suddenly present investors with "special charges."

"When it comes to M&A - management promises to the contrary - the whole is often less than the sum of the parts," analysts at S&P Global led by Richard Tortoriello wrote.

"The larger the deal size relative to the acquirer and the more stock consideration paid, the greater the subsequent underperformance." 

This accords reasonably well with earlier work by Sandra Mortal of the University of Memphis and Michael Schill of the University of Virginia who found a connection in stock deals between poor performance and higher rates of asset growth.

Firms, it would seem, wishing to expand their balance sheets often use stock over cash to fund expansion but find it difficult to pull off that growth profitably.

In the S&P Global sample acquirers with the highest pre-acquisition asset growth lag the market by 5.8 percent one year after the deal and by 13.3 percent over three years.

Earlier studies have also, on the whole, found evidence that mergers don't create value for owners.