We model in an endogenous growth set-up the hypotheses that the expansion of market activities weakens social capital formation and that firms can invest in formal mechanisms of control and enforcement to substitute for social capital (trust, work ethics, honesty). The model shows that the economy tends to grow faster when it is relatively poorer in social capital and that perpetual growth can be consistent with the progressive erosion of social capital. These results may help to reconcile Putnam's claim that social capital has declined in the U.S. with the satisfactory growth performance of the U.S. over the same period.