1 + 1 = 1.5?

Corporate mergers in recent years have not worked terribly well, according to a KPMG study reported in Financial Times.

...two thirds of the companies bought between 1996 and 1998 still needed to be properly integrated.

Its original study of the 500 largest cross-border deals struck during this period found most had destroyed value for shareholders in the short term. It has now returned to these companies and found a higher-than-expected proportion are trying to turn the clock back by selling businesses acquired during the boom.

Mergers are supposed to enhance value. The cliche for synergy is that "1+1=3." But the studies seem to always show that 1+1=1.5

Discussion Question. Does one have to resort to behavioral economics (i.e., systematic irrationality) to explain why value-destroying mergers take place?