Four explanations for secular stagnation are distinguished: a rise in global saving, slow population growth that makes investment less attractive, averse trends in technology and productivity growth, and a decline in the relative price of investment goods. A long view from economic history is most supportive of these four views.
The same investment projects can be pursued, it is hypothesized, by committing a smaller share of GDP, and any additional projects that might be rendered attractive by this lower cost of capital are not enough to offset the decline in the investment share. With less investment spending chasing the same savings, the result can be lower real interest rates and, potentially, a chronic excess of desired saving over desired investment.
He has this graphic which highlights the declining relative price of investment goods.
While this may be true of many developed economies, where the services sector predominates, it may be less so with developing economies. In these countries, manufacturing still makes up a significant share of the GDP and services a less dominant one. This is one more reason for appreciating the international dimension of secular stagnation hypothesis. Once we assume an open economy, the potential for mutually beneficial outcomes from international trade and cross-border capital flows are immense.