Let my people go (home) to Spain: a genealogical model of Jewish identities since 1492

Abstract

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, that is, 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 transmissible "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 genealogical 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.

Figure 1. Genealogical history with mating preferences.

Each panel illustrates the ancestors of two focal individuals given and (top-middle-bottom). Initially, at , there are 20% type-1 individuals (red circles) and 80% type-2 individuals (blue circles). Ancestors are denoted by red and blue lines, respectively. When viewing the process retrospectively from the present-day (top line in each panel, ), it is apparent that all ancestors share the same identity as the focal two individuals when . However, when then one (or more) ancestors may have a different identity than that of the focal individual.

Figure 2. Rapid increase in frequency of genealogical link to a previously minority identity.

Solid lines with symbols denote theory of Eq. (1) (colored based on initial frequency of type-1 individuals) while circles denote simulation. The simulated population has individuals.

Figure 3. Nearly all modern-day Jews are likely to have many ancestors expelled from Spain in 1492.

Genealogical dynamics are simulated using in a population of initial size at generation 0 that is of size at generation 20. The choice of corresponds to a relative preference of in-group mating of 1000∶1 relative to out-group mating. (A) Population-state of identities, , where the identities (red for Type-1, , and blue for Type-2, ) are displayed for each individual (column) in each generation (row). (B) Population-state of genealogical identities, , where the identities (red for and blue for ) are displayed for each individual (column) in each generation (row). (C) Comparison of theoretical prediction (red circles) of the fraction of individuals with at least one Sephardic ancestor, , with simulations (all variation contained in black shaded region). The identity information in panels (A) and (B) are downsized by a factor of 1000∶1 for the purposes of visualization.

Figure 4. Individuals have many ancestors with different identities than their own.

The results are from a simulation of a population with , , and . (Left) Fraction of ancestors in that are type 1, for individuals who at generation self-identify as type 1 (red) or type 2 (blue); (Right) Fraction of individuals with at least one type-1 ancestor, . Generations increase from to along the y-axis in both panels.