For many years, economists had little interest in what happened inside firms, preferring instead to examine the workings of the different sorts of industries in which firms operate, ranging from perfect competition to monopoly. Since the 1960s, however, sophisticated economic theories of how firms work have been developed. These have examined why firms grow at different rates and tried to model the normal life cycle of a company, from fast-growing start-up to lumbering mature business. The aim is to explain when it pays to conduct an activity within a firm and when it pays to externalize it through short- or long-term arrangements with outsiders, be they individuals, exchanges or other companies. The theories also look at the economic consequences of the different incentives influencing individuals working within companies, tackling issues such as pay, agency costs and corporate governance structures.