It has been reported for a long time that about 10% of all putative fathers are not the biological fathers. I have had my doubts that such a high prevalence is general. However, no good data has been available in Germany. However, researchers from the University of Duesseldorf (M. Wolf et al. (2012) Estimating the prevalence of nonpaternity in Germany. Human Nature, 23, 208-217) could recently and convincingly show that the prevalence in Germany these days is below 1%. These results are comparable with those from England and Switzerland.
The 10% cuckold children is a modern myths which originated in the 1950s and has been reported in the yellow press, but also in scientific publications. The prevalence of paternity discrepancy was possibly higher than 1% in earlier times. These days fathers ought to have less doubt whether they are the real biological father. Even if they have doubts about the fidelity of their partners at the time of conceptions, in most cases it turns out that they are the biological father nevertheless.