NORKUS, Zenonas. On Positive and Negative Altruism. Problemos, [S. l.], v. 47, p. 15–39, 1995. DOI: 10.15388/Problemos.1995.47.7035. Disponível em: https://www.zurnalai.vu.lt/problemos/article/view/7035. Acesso em: 2 jan. 2026.