Steve Jobs, Apple’s ailing CEO, is scheduled to return to work this month after a six-month leave, but investors are feeling skittish. Every time he sneezes, shares of Apple catch a cold. Can a CEO—even one as talented and visionary as Jobs—really make or break a corporation? Many business scholars have grown skeptical of the idea of chief executive as superhero. Cutting-edge research reveals that while some CEOs clearly do make a big difference, many are merely the most visible cogs in complex machines.