Extrinsic Risk

Studies of human life history are focused on two fundamental decisions: to reproduce now or later, and quantity versus quality—how many offspring to produce and how much to invest in each (Borgerhoff-Molder 1992; Chisholm 1999; Hill and Kaplan 1999; Stearns 1992). Life history theory describes the timing of life course events (e.g., age at sexual maturity, birth spacing, length of parental investment) as adaptive, species-typical responses shaped by natural selection and as phenotypically variable responses of individuals within and between populations, given a speciestypical range (Ellis et al. 2009). The papers contained in this special edition of Human Nature draw on theoretical developments exploring human behavioral diversity in life-history-shaping decisions within and between populations in response to various environmental conditions. Here, the environmental conditions of interest are conceptualized as components of extrinsic risk. Extrinsic risk is any unavoidable ecological factor that reduces an offspring’s reproductive value and is immune to reduction through enhanced parental investment (Quinlan 2006). Life history studies on extrinsic risk have typically focused on the effect of extrinsic mortality (e.g., Pennington and Harpending 1988); however, more recently the importance of non-lethal risks has also been elaborated (see Ellis et al. 2009 for review of extrinsic morbidity-mortality). The theme of this special edition grew from two papers exploring extrinsic risk and behavior (Quinlan 2010; Schechter and Francis 2010) originally presented at an invited session organized by Brooke Scelza and Dawn Neill, and sponsored by the Evolutionary Anthropology Society, at the 2008 meetings of the American Anthropological Association. The session, entitled, “Evolutionary Perspectives on Hum Nat (2010) 21:99–102 DOI 10.1007/s12110-010-9083-3