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etymonline.com
"altruism (n .)". Online Etymology Dictionary. Douglas Harper. Retrieved 19 September 2014.
Zahavi, A. (1995). "Altruism as a handicap – The limitations of kin selection and reciprocity". Avian Biol. 26 (1): 1–3. doi:10.2307/3677205. JSTOR3677205.