The War

The key seems to have been Genghis Khan's unique value system:

"The greatest joy a man can know is to conquer his enemies and drive them before him. To ride their horses and take away their possessions. To see the faces of those who were dear to them bedewed with tears, and to clasp their wives and daughters in his arms"

Preferring rape and conquest to hunting and falconry, coupled with building an empire and "a social legacy that benefited his sons' sons unto the seventh generation and even beyond", meant that Genghis' progeny multiplied explosively, and his apparent Y-chromosome lineage today features prominently in the population genetics of Asia.

In "The Genetic Legacy of the Mongols" (Abstract|PDF), to be featured in the March 2003 issue of The American Journal of Human Genetics but which has already been published electronically, the authors report their discovery of the aforementioned Y-chromosome lineage, which due to it's age (~1,000 years), place of origin (Mongolia), and rapid spread, must in all probablity be associated with Genghis Khan or one of his immediate forebears.

Though absolute proof that the lineage in question is Genghis Khan's awaits the recovery of his remains and successful sequencing of his DNA, the only other possible explanation is that Genghis Khan did not spread his genes while some unknown man living in the same place and time did. This is unlikely, to say the least, since the enormous reproductive success of Genghis Khan's descendants is well attested in the historical record.

In fact, as we learn in Steve Sailer's UPI write-up of the study ("Genes of history's greatest lover found?"):

Incredibly, as late as the early 20th century, three-quarters of

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