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...vers 1974) has profound effects on offspring survival (Hill & Hurtado 1996; Pavard et al. 2005) and reproductive success (Lindström 1999; Lummaa 2003). However, parental investment is not unlimited (=-=Stearns 1992-=-; Kaplan 1996). The potential fitness benefits of investment in different offspring are seldom equal, potentially leading to selection pressures on parents to invest differentially in their offspring ...

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...ment of economic games opens the possibility of objectively measuring cooperation. We used the investment game, a tool to study trust (or risk taking) and positive reciprocity in social interactions (=-=Berg et al. 1995-=-). We investigated whether and how behaviour in an investment game played with nonrelated adults is affected by birth order. METHODS The Investment Game We used an investment game based on the protoco...

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...ed students. The These results constitute some of the first experimental evidence that birth order differences established huma ts (Feh 006). C can on tor tha where no direct reciprocity is possible (=-=Nowak & Sigmund 1998-=-; Fehr & Gachter 2002; Fehr & Fischbacher 2003; Fehr 2004; Boyd 2006). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (Hill ...

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... results constitute some of the first experimental evidence that birth order differences established huma ts (Feh 006). C can on tor tha where no direct reciprocity is possible (Nowak & Sigmund 1998; =-=Fehr & Gachter 2002-=-; Fehr & Fischbacher 2003; Fehr 2004; Boyd 2006). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (Hill & Hurtado 1996; Pavar...

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...a where no direct reciprocity is possible (Nowak & Sigmund 1998; Fehr & Gachter 2002; Fehr & Fischbacher 2003; Fehr 2004; Boyd 2006). Examples include blood donation and charity, but also investment (=-=Trivers 1974-=-) has profound effects on offspring survival (Hill & Hurtado 1996; Pavard et al. 2005) and reproductive success (Lindström 1999; Lummaa 2003). However, parental investment is not unlimited (Stearns 1...

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... 1998; Fehr & Gachter 2002; Fehr & Fischbacher 2003; Fehr 2004; Boyd 2006). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (=-=Hill & Hurtado 1996-=-; Pavard et al. 2005) and reproductive success (Lindström 1999; Lummaa 2003). However, parental investment is not unlimited (Stearns 1992; Kaplan 1996). The potential fitness benefits of investment i...

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... evier .com/locate/anbehavd by Elsevier Ltd. All rights reserved. ehavoffspring of different ages and developmental stages (Keister 2003; Gurven & Walker 2006), which exacerbates sibling competition (=-=Mock & Parker 1997-=-). Sulloway (1996) argued that many psychological differences between siblings result from different adaptive strategies based on differential access to parental investment and, more specifically, on ...

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...nefits to the actors’ kin). In animals, cooperative behaviours are almost exclusively restricted to kin groups, apart from rare and specific cases of repeated encounters between pairs of individuals (=-=Dugatkin 1997-=-; Fehr & Fischbacher 2003). In humans, individuals cooperate in large groups involving nonrelatives, in situations Most studies on cooperative behaviours report important interindividual variation wit...

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...) 1405–1411E-mail address: alexandre.courtiol@univ-montp2.fr (A. Courtiol).the cooperative action (Clutton-Brock 2002; Fehr & Fischbacher 2003; Lehmann & Keller 2006; Nowak 2006; Lehmann et al. 2007; =-=West et al. 2007-=-). Therefore, cooperation can evolve and spread in populations provided that it entails either direct benefits to the actors or indirect benefits (through an increase in inclusive fitness, i.e. benefi...

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... profound effects on offspring survival (Hill & Hurtado 1996; Pavard et al. 2005) and reproductive success (Lindström 1999; Lummaa 2003). However, parental investment is not unlimited (Stearns 1992; =-=Kaplan 1996-=-). The potential fitness benefits of investment in different offspring are seldom equal, potentially leading to selection pressures on parents to invest differentially in their offspring (Biermann & R...

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... access to parental investment and, more specifically, on the microenvironment defined by birth order. Firstborn, middleborn or lastborn status affects the relative competitive ability of each child (=-=Sulloway 1996-=-). During childhood, a firstborn has an advantage merely because he/she is older, and thus physically stronger and cognitively more advanced than laterborn children. In addition, at a given time, the ...

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... blood concentrations of hormones such as cortisol and testosterone (Flinn & England 1997; Alvergne et al. 2008). In addition, these hormones have been found to influence behaviour in economic games (=-=Burnham 2007-=-; Apicella et al. 2008; Coates & Herbert 2008). Implications for the Evolution of Human Cooperation The role of sibling competition in determining adult cooperation may provide some indirect insights ...

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...utionary psychologists, birth order effects result from sibling competition for parental investment (Sulloway 1996), together with an unequal distribution of investment among siblings by the parents (=-=Hertwig et al. 2002-=-). Hence, interindividual variation in cooperation partly results from sibling competition. Firstborns appeared to be less trustful and reciprocate less than laterborn and only children. The absence o...

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...st studies on cooperative behaviours report important interindividual variation within populations. Empirical studies of this variation are essential to understand the evolution of human cooperation (=-=Zahavi 1995-=-; Lotem et al. 1999; McNamara et al. 2004). Nevertheless, studies of this nature are still scarce (Scheres & Sanfey 2006). A few studies have related interindividual variability to sex and age (e.g. M...

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...adulthood, as they have already survived the first years of life when mortality is highest; and (2) they are likely to start reproducing earlier, thereby shortening the generation time (Trivers 1974; =-=Voland 1998-=-; Draper & Hames 2000). If parents are going to invest differentially in offspring, they should thus increase investment in firstborns from which fitness benefits are likely to be greatest (Jeon 2008)...

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... possible proximal mechanisms, the endocrine system is likely to be involved. The familial environment has long-term effects on the blood concentrations of hormones such as cortisol and testosterone (=-=Flinn & England 1997-=-; Alvergne et al. 2008). In addition, these hormones have been found to influence behaviour in economic games (Burnham 2007; Apicella et al. 2008; Coates & Herbert 2008). Implications for the Evolutio...

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... order can shape adult cooperative le at ScienceDirect haviour evier .com/locate/anbehavd by Elsevier Ltd. All rights reserved. ehavoffspring of different ages and developmental stages (Keister 2003; =-=Gurven & Walker 2006-=-), which exacerbates sibling competition (Mock & Parker 1997). Sulloway (1996) argued that many psychological differences between siblings result from different adaptive strategies based on differenti...

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...rations of hormones such as cortisol and testosterone (Flinn & England 1997; Alvergne et al. 2008). In addition, these hormones have been found to influence behaviour in economic games (Burnham 2007; =-=Apicella et al. 2008-=-; Coates & Herbert 2008). Implications for the Evolution of Human Cooperation The role of sibling competition in determining adult cooperation may provide some indirect insights into the evolutionary ...

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...ce that birth order differences established huma ts (Feh 006). C can on tor tha where no direct reciprocity is possible (Nowak & Sigmund 1998; Fehr & Gachter 2002; Fehr & Fischbacher 2003; Fehr 2004; =-=Boyd 2006-=-). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (Hill & Hurtado 1996; Pavard et al. 2005) and reproductive success (Lindst...

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...nteract with nonkin children during childhood). This is supported by studies of familial attachment: for instance, firstborns feel closer to their parents and other relatives than laterborn children (=-=Salmon & Daly 1998-=-). In addition, our study showed a nonsignificant trend for middleborn children to be the most trustful towards nonkin and to reciprocate the most. This is consistent with the fact that middleborn chi...

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...ntal evidence that birth order differences established huma ts (Feh 006). C can on tor tha where no direct reciprocity is possible (Nowak & Sigmund 1998; Fehr & Gachter 2002; Fehr & Fischbacher 2003; =-=Fehr 2004-=-; Boyd 2006). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (Hill & Hurtado 1996; Pavard et al. 2005) and reproductive succ...

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... cooperative behaviours report important interindividual variation within populations. Empirical studies of this variation are essential to understand the evolution of human cooperation (Zahavi 1995; =-=Lotem et al. 1999-=-; McNamara et al. 2004). Nevertheless, studies of this nature are still scarce (Scheres & Sanfey 2006). A few studies have related interindividual variability to sex and age (e.g. Murnighan & Saxon 19...

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...between birth order and personality traits (Sulloway 1995). They can sometimes providemore or less direct information on cooperativeness because several personality traits are associated with it (see =-=Jefferson et al. 1998-=- and references therein; Paulhus et al. 1999; Sulloway 2001 and references therein). Overall, results reported in these studies appear relatively inconsistent. Moreover, these studies used subjective ...

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...al Behaviour 78 (2009) 1405–1411E-mail address: alexandre.courtiol@univ-montp2.fr (A. Courtiol).the cooperative action (Clutton-Brock 2002; Fehr & Fischbacher 2003; Lehmann & Keller 2006; Nowak 2006; =-=Lehmann et al. 2007-=-; West et al. 2007). Therefore, cooperation can evolve and spread in populations provided that it entails either direct benefits to the actors or indirect benefits (through an increase in inclusive fi...

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...lloway 1995). They can sometimes providemore or less direct information on cooperativeness because several personality traits are associated with it (see Jefferson et al. 1998 and references therein; =-=Paulhus et al. 1999-=-; Sulloway 2001 and references therein). Overall, results reported in these studies appear relatively inconsistent. Moreover, these studies used subjective measures of cooperation (peer- or self-repor...

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...espite using larger samples. In addition, our analyses also reveal two problems related to the use of students in empirical work on cooperation. First, as in many studies based on university samples (=-=Altus 1966-=-; Rohde et al. 2003), firstborns are overrepresented compared to lastborn children. Parental resource depletion and different abilities of siblings are the classical explanations for this phenomenon (...

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...ours report important interindividual variation within populations. Empirical studies of this variation are essential to understand the evolution of human cooperation (Zahavi 1995; Lotem et al. 1999; =-=McNamara et al. 2004-=-). Nevertheless, studies of this nature are still scarce (Scheres & Sanfey 2006). A few studies have related interindividual variability to sex and age (e.g. Murnighan & Saxon 1998; Andreoni & Vesterl...

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...hanisms, the endocrine system is likely to be involved. The familial environment has long-term effects on the blood concentrations of hormones such as cortisol and testosterone (Flinn & England 1997; =-=Alvergne et al. 2008-=-). In addition, these hormones have been found to influence behaviour in economic games (Burnham 2007; Apicella et al. 2008; Coates & Herbert 2008). Implications for the Evolution of Human Cooperation...

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...studies of this variation are essential to understand the evolution of human cooperation (Zahavi 1995; Lotem et al. 1999; McNamara et al. 2004). Nevertheless, studies of this nature are still scarce (=-=Scheres & Sanfey 2006-=-). A few studies have related interindividual variability to sex and age (e.g. Murnighan & Saxon 1998; Andreoni & Vesterlund 2001; Solnick 2001), but results are often inconsistent, and a large part o...

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... 2002; Fehr & Fischbacher 2003; Fehr 2004; Boyd 2006). Examples include blood donation and charity, but also investment (Trivers 1974) has profound effects on offspring survival (Hill & Hurtado 1996; =-=Pavard et al. 2005-=-) and reproductive success (Lindström 1999; Lummaa 2003). However, parental investment is not unlimited (Stearns 1992; Kaplan 1996). The potential fitness benefits of investment in different offsprin...

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...ical values from tables used in nonparametric tests. Consequently, as suggested by Manly (2007), two statistical analyses were performed based on randomization tests: test 3 (Manly 1995) and test DO (=-=Manly & Francis 1999-=-). These two tests allow one to test differences between means of several samples in iour 78 (2009) 1405–1411the situation where the samples may be drawn from distributions net 3 buti ehavwith differe...

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...n this study we tested the hypothesis that birth order can shape adult cooperative behaviours in a similar way. Many studies have examined the relationship between birth order and personality traits (=-=Sulloway 1995-=-). They can sometimes providemore or less direct information on cooperativeness because several personality traits are associated with it (see Jefferson et al. 1998 and references therein; Paulhus et ...

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... Kaplan 1996). The potential fitness benefits of investment in different offspring are seldom equal, potentially leading to selection pressures on parents to invest differentially in their offspring (=-=Biermann & Robertson 1983-=-; Slagsvold 1997; Jeon 2008). In humans, parents need to divide their limited resources between several simultaneously dependent * Correspondence: A. Courtiol, CNRS, Institut des Sciences de l’Evoluti...

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...rect estimate of the critical values from tables used in nonparametric tests. Consequently, as suggested by Manly (2007), two statistical analyses were performed based on randomization tests: test 3 (=-=Manly 1995-=-) and test DO (Manly & Francis 1999). These two tests allow one to test differences between means of several samples in iour 78 (2009) 1405–1411the situation where the samples may be drawn from distri...