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I present argument and evidence for a structural ecology of social capital that describes how the value of social capital to an individual is contingent on the number of people doing the same work. The information and control benefits of bridging the structural holes-or, disconnections between nonredundant contacts in a networkthat constitute social capital are especially valuable to managers with few peers. Such managers do not have the guiding frame of reference for behavior provided by numerous competitors, and the work they do does not have the legitimacy provided by numerous people doing the same kind of work. I use network and performance data on a probability sample of senior managers to show how the value of social capital, high on average for the managers, varies as a power function of the number of people doing the same work.'
Some people enjoy higher incomes than others. Some are promoted faster. Some are leaders on more important projects. The human capital explanation is that inequality results from differences in individual ability. The usual evidence is on general populations, as is Becker's (1975) pioneering analysis of income returns to education, but the argument is widely applied by senior managers to explain who gets to the top of corporate America-managers who make it to the top are smarter or better educated or more experienced. But, while human capital is surely necessary to success, it is useless without the social capital of opportunities in which to apply it.
Social capital can be distinguished in its etiology and consequences from human capital (e.g., Coleman, 1990; Bourdieu and Wacquant, 1992; Burt, 1992; Putnam, 1993; Lin, 1998). With respect to etiology, social capital is a quality created between people, whereas human capital is a quality of individuals. Investments that create social capital are therefore different in fundamental ways from the investments that create human capital (Coleman, 1988, 1990). I focus in this paper on consequences, a focus in network analysis for many years (Breiger, 1995). With respect to consequences, social capital is the contextual complement to human capital. Social capital predicts that returns to intelligence, education, and seniority depend in some part on a person's location in the social structure of a market or hierarchy. While human capital refers to individual ability,...