After a summmary of the theoretical background of their research, the authors report two cases: two families of former agricultural workers in the region of Bordeaux, confronted with the French principle of “obligation alimentaire” [family support duty] towards the elderly: two old ladies, who previously lived with a daughter or grand-daughter, are placed under care — the other children being then required by a judge to contribute to the payment of such care. The survey allows to clarify the principles at the root of observed behaviors. Beyond the republican principle of equality among children, the birth rank, gender, past relationships with the dependent individual play a part in the making of a vocation to care, as well as in conflicts triggered by the calling into question of routines. The suggested mathematical formalization, which shall be applied to data collected thanks to a specially designed survey, allows to assess the relative weight of each family member during decision making.