Clark studied the genetic changes in the English population on the basis of surnames. In a few hundred years the surnames of lower/criminal class have disappeared, replaced by descendants of the upper/richer classes. In our generation, it seems to work on the reverse.
Why this happened in South East England and not in Bulgaria? England was/is a stable, well-governed society for centuries, where violence does not pay. Bulgaria was a frontier lawless region, where violence and brutality was survival necessity.