Investment can sustainably drive growth only up to the point when returns decline dramatically. In the case of public-sector investment, that means that the present value of the increment to the future GDP path (using a social discount rate) is greater than the investment itself.
当投资回报率大幅下降时，投资便无法再持续地推动增长。在公共部门投资方面，转折点发生在未来GDP路径增量的现值（用社会贴现率计算）大于投资本身时。