ways (Pfeiffer et al 2001), theoretical approaches to immunology (Nowak et al 1991, 1995), and the study of animal and human behavior (Parker 1974, Enquist & Leimar 1983, McNamara & Houston 1986, Milinski 1987, Fudenberg & Tirole 1991, Binmore 1994, Hammerstein Sc Selten 1994, Ohtsuki & Iwasa 2004, Nowak St Sig