The size and the development of the shadow economy of seventy-six countries has been shown in order to clarify the importance of broad reforms of the tax and social security systems. For this, we have introduced several methods of measuring the extent of illicit work, even though this is a difficult task. Not one of the applied approaches allows for exact calculations. All of these are subject to considerable flaws and, therefore, can only give crude clues to the size of the black markets. However, by comparing the results, it can be shown that there is a similar tendency in all the various methods. The shadow economy has gained considerable momentum from 1970 to the end of the 1990s in many countries. In OECD countries, its size has nearly doubled and lies between 10 and 20% in the years 1990–7, whereas it was between 1 and 5% during the period from 1970 to 1980. Poll results confirm that illicit activities are increasingly accepted and that willingness to take up an illegal occupation is rising.

In the context of an integrated, socio-scientific approach, various theoretical methods were introduced with which the cause and effects of rising shadow economic activities can be examined. Here, the objective was to underline the possibilities and synergy effects of combining economic, socio-psychological, and sociological theories. Based on rational choice theory, these research areas can be integrated into an interdisciplinary model.