Low interest rates are not sufficient reason in themselves to increase public investment

It is increasing claimed that governments should take advantage of the very low interest rates to increase public investment. This concerns the euro zone in particular. But a more nuanced analysis is needed: Low interest rates make it is easier to carry out public investments, but also private investments. If the marginal return on private capital is high, then it would be better to increase private investment instead; If public investments offer high economic and social returns thanks to large externalities (financing the energy transition, companies in industries of the future, education and research, etc.), they should be made even if government bond yields were higher than today; If public investments offer a low return, but which has now become higher than interest rates, because interest rates are very low, governments must be cauti ous , as a rise in interest rates in the future would severely worsen the situation of public finances.