High paternity certainties of Jewish priests

Recent evidence (Skorecki et al. 1997) of significant differences between the Y chromosomes of Jewish priests (Kohanim) and laymen (Israelites) indicate very high degrees of paternity certainty (p, the probability that a man's wife's children are his own, Hartung 1985). Estimates of gene flow into the Kohanim from Israelites indicate that approximately 20% of living Ashkenazic Kohanim and 56% of living Sephardic Kohanim are descended through males from the founding priest. These suggest average per generation paternity certainties of .988 and .996 respectively. Multinomial discrete generation simulation establishes fairly narrow 95% lower bounds on these estimates. Given that paternity certainty is thought to vary with cultural practices (Hartung 1985), the genetic evidence poses the important social and cultural question of how these high certainties were achieved.