Post-Keynesian economics also concentrates on macroeconomic rigidities and adjustment processes. Search costs can contribute to real rigidities through "thick market externalities". These rigidities do not allow flexible responses to present time exigencies. Real rigidities, along with nominal, are a key part of new Keynesian economics. Capital market imperfections lead to more real rigidities. Rigidities are seen in the institutional arrangements, or in the monopoly power. They did not anticipate, however, the rigidities of the Stalinist rule that followed. "Customer markets" can also create real rigidities. When it is past success and attention, it creates core rigidities.