Recent research on kin investment shows a matrilateral bias as a function of paternity uncertainty. Kin investment, however, is a special case of kin altruism. We thus hypothesize that psychological adaptations have evolved to regulate cousin-directed altruism according to predictably variable levels of paternity uncertainty in different categories of cousins. We develop a formal mathematical model that predicts that individuals should be most willing to act altruistically towards their mother's sister's (MoSis) children and least willing to act altruistically towards their father's brother's (FaBro) children. Altruism towards father's sister's (FaSis) and mother's brother's (MoBro) children are predicted to fall in between. An empirical study ( N =195), assessing expressed altruistic proclivities, confirmed the predictions from the model. Participants expressed willingness-to-help following the descending order: (i) MoSis children, (ii) MoBro children, (iii) FaSis children, and (iv) FaBro children. The psychological variables of emotional closeness, empathic concern and contact frequency showed precisely the same pattern across distinct cousins, providing convergent confirmation of the model. The results support the hypothesis of cousin-specific adaptations sensitive to varying probabilities of paternity uncertainty.