A MERGER between companies from unrelated sectors. Conglomerate mergers, which were popular during the 1960s and 1970s, have been relatively rare since many unsuccessful deals were dismantled in the 1980s. Academic and empirical evidence supports the idea that it is more efficient for an investor to hold a diversified PORTFOLIO of COMMON STOCKS, than for a company to merge with, or acquire, a diversified group of companies. See also HORIZONTAL MERGER, VERTICAL MERGER.