Why is capital productivity declining?

When we calculate the ratio of real GDP to real total net capital or net capital excluding housing for OECD countries, we see that capital productivity has been declining since 1990. There is a debate on the measurement of quality effects and their impact on the capital goods deflator. When we calculate the ratio of nominal GDP to nominal total net capital or net capital excluding housing, we also find that capital productivity is declining. This is all the more astonishing as: The weight of industry, which is more capital intensive, has diminished; The degree of corporate automation has increased; R&D has increased. What can account for the decline in capital productivity? Increased need for certain types of capital (intellectual property capital: patents, software, office capital, industrial and logistics buildings, etc.)? A substitution of capital for labour? Over-accumulation of capital? The facts are consistent with over-accumulation of capital and increased investment in intellectual property, but not with a substitution of capital for labour.