Fertility Theory: Theory of Life History Evolution Gert Stulp University of Groningen Department of Sociology Grote Rozenstraat 31 9712 TS Groningen The Netherlands Phone: 0031(0)50-363 6326 Fax: 0031(0)-50 363 6304 Email: g.stulp@rug.nl Louise Barrett University of Lethbridge Department of Psychology 4401 University Drive West Alberta T1K 3M4 Canada UNISA Applied Behavioural Ecology and Ecosystems Research Unit, Private Bag X6 Florida 1710 Johannesburg South Africa Phone: 403-317-5039 Fax: 403-329-2555 Email: louise.barrett@uleth.ca To appear in: The International Encyclopedia of the Social and Behavioral Sciences, 2nd Ed (Ed. J. Wright). Elsevier, Amsterdam