 A number of mechanisms have been shown to support the emergence of cooperation: kin selection (Fisher 1930; Haldane 1932), green beards (Hamilton 1964a,b; Dawkins 1976), secret handshakes (Robson 1990; Wiseman &amp; Yilankaya 2001), iterated games, spatialized and structured interactions (Grim 1995; Skyrms 1996, 2004; Grim, Mar, &amp; St. Denis 1998; Alexander 2007), and noisy signals (Nowak &amp; Sigmund 1992). This section offers examples of the last two of these.