Corporate dealmakers should heed the lessons of past merger waves … WHY mergers and acquisitions (M&A) come in waves is not fully understood. Companies’ fortunes are affected by the economy’s ebb and flow, but this does not seem enough to explain why merger activity crests and breaks so dramatically. Yet crest and break it does. In America there have been at least five merger waves, in which the number of deals swelled, peaked then tumbled. The first, in the 1920s, ended with the onset of the Great Depression. It was less obvious why the following ones—in the 1960s and then in each decade since 1980—were so strong. Now, say some experts, a powerful sixth wave is forming.