Big Pharma can't resist big mergers. Here's why.

Pharmaceutical companies began disappearing roughly 30 years ago—an early milestone was the Bristol-Myers merger with Squibb in 1989. Since then, drug companies have been among the heaviest users of mergers and acquisitions in Corporate America, whittling down the number of traditional companies (counterbalanced, of course, by dense flocks of biotech start-ups), as the survivors grew larger, more complex, and increasingly harder to grow.