  The Spanish government recently announced an official fast-track path to
citizenship for any individual who is Jewish and whose ancestors were expelled
from Spain during the inquisition-related dislocation of Spanish Jews in 1492.
It would seem that this policy targets a small subset of the global Jewish
population, i.e., restricted to individuals who retain cultural practices
associated with ancestral origins in Spain. However, the central contribution
of this manuscript is to demonstrate how and why the policy is far more likely
to apply to a very large fraction (i.e., the vast majority) of Jews. This claim
is supported using a series of genealogical models that include transmissable
"identities" and preferential intra-group mating. Model analysis reveals that
even when intra-group mating is strong and even if only a small subset of a
present-day population retains cultural practices typically associated with
that of an ancestral group, it is highly likely that nearly all members of that
population have direct geneaological links to that ancestral group, given
sufficient number of generations have elapsed. The basis for this conclusion is
that not having a link to an ancestral group must be a property of all of an
individual's ancestors, the probability of which declines (nearly)
superexponentially with each successive generation. These findings highlight
unexpected incongruities induced by genealogical dynamics between present-day
and ancestral identities.
