Pielou, E.C. (1966). "The measurement of diversity in different types of biological collections". Journal of Theoretical Biology. 13: 131–144. doi:10.1016/0022-5193(66)90013-0. Information content of collections of biological species.
Hamilton, W.D. (1971). "Geometry for the selfish herd". Journal of Theoretical Biology. 31 (2): 295–311. doi:10.1016/0022-5193(71)90189-5. PMID5104951. An antithesis to the view that gregarious behaviour is evolved through benefits to the population or species.