Examples of positive externalities include the spillovers of benefits which occur when firms train workers, invest in research and development and adopt new technologies which generate “learning by doing’ benefits. Examples of negative externalities include pollution or collusive or abusive manipulation of markets. Given that investable resources are scarce, long term development requires that social rates of return are high.
Hence rapid economic development is more likely to be realised in a society which can enable the private sector to generate high private 2  rates of return to investment while at the same time yielding positive externalities so that social rates of return are even higher.