The existence of a bastardy-prone sub-society that is selected against would have a long-term effect on population structure. A bastardy-prone sub-society has been identified in a small population isolate in the French West Indies, based on surname analysis and correlations between related women bearing illegitimate children. Data analysis indicates that membership in this bastardy-prone sub-society results in reduced fertility and increased infant mortality. The aggregate effect of decreased reproductive success for mothers of illegitimate children and their daughters, together with a higher death rate for illegitimate children, could lead to an inverse pyramidal effect on family size over generations, resulting in possible family extinction.