On intrapersonal reciprocityDavid Haig*Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street,Cambridge, MA 02138, USAReceived 26 June 2003; received in revised form 5 August 2003AbstractThe existence of conflicts between different sets of genes within the genome is now widelyaccepted. But where there is conflict, there are also benefits to be gained from cooperation between thecontending parties to reduce conflict costs. The potential for reciprocal altruism (Trivers, 1971) withinan individual organism has hitherto attracted little attention but raises the possibility of complexinteractions within the self.D 2003 Elsevier Inc. All rights reserved.Keywords: Internal conflict; Genomic imprinting; Reciprocity; Prisoner’s Dilemma1. IntroductionIntrospection suggests that I often attempt to modify my own behavior by an internallyvoiced mixture of exhortation, bribes, and threats. Such intrapersonal cajolement has limitedeffectiveness, in part, because threats and promises to myself lack credibility. If I renege on acontract with myself, who will enforce sanctions? If I offer myself a reward now—for latergood behavior—why am I bound to fulfil the second half of the bargain? Is it credible that Iwould knowingly do harm to myself to punish a past transgression?The existence of intrapersonal persuasion poses a philosophical conundrum. I know what Iknow, and I know what I want. So, why do I need to persuade myself of anything? One mightargue that internal bargaining is simply a reuse, or misapplication, of tools that are effective inthe control of others’ behavior to self-control. I modify my behavior in response to the threats1090-5138/03/$ – see front matter D 2003 Elsevier Inc. All rights reserved.doi:10.1016/S1090-5138(03)00063-1* Tel.: +1-617-496-5125; fax: +1-617-495-5667.E-mail address: [email protected] (D. Haig).Evolution and Human Behavior 24 (2003) 418–425and bribes of others and, in turn, use threats and bribes to modify others’ behavior. Why notuse these same methods—of proven effectiveness—on myself? Or, one might argue thatintrapersonal bargaining resembles interpersonal bargaining because the self containsmultiple agents with different, sometimes conflicting, agendas. It is this possibility—of aninternally complex self in which there is potential for strategic negotiation betweencontending parties—that I will explore in this note. In particular, I will address the possibilityof intrapersonal reciprocity.2. An internal conflictEvolutionary theory has revealed the possibility of conflicts within organisms that arisewhen different replicators have different rules of transmission (Hurst, Atlan, & Bengtsson,1996). Could these conflicts underlie problems of self-control? The potentially competingagendas within the self, both genetic and memetic, are complex. Therefore, to focus discussionit will be useful to consider a simple exemplar of an internal genetic conflict. For this purpose Iwill consider a conflict between the genes that an individual (call him Bob, in honor of RobertTrivers) inherits from his mother and his father.Suppose that Bob’s parents divorce and remarry, and that his mother and father each have achild in their new marriages. Bob thus has a maternal half-sib (Maddy) and a paternal half-sib(Paddy). What cost (C ) would Bob’s genes be prepared to accept in exchange for a benefit (B)to Maddy? Bob’s maternally derived (henceforth madumnal) genes have one chance in two ofbeing transmitted to Maddy. Therefore, madumnal alleles would favor Maddy receiving thebenefit, as long as its value exceeded twice the cost to Bob (B >2C ). However, Bob’spaternally derived (henceforth padumnal) genes are absent from Maddy. For these genes, nobenefit—no matter how great—to Maddy could justify any cost—no matter how small—toBob (i.e., C < 0 for the exchange to be profitable). Bob’s madumnal and padumnal genomes arein conflict whenever B >2C > 0. A similar conflict exists over benefits that Bob could conferon Paddy, but in this case the roles of madumnal and padumnal genomes are reversed.3. Conflict resolutionIn what ways could Bob’s internal conflict be resolved? The traditional answer invokes a‘‘veil of ignorance’’ (Rawls, 1971). If genes lack information about their parental origin, theyare constrained to behave in the same manner when inherited from either parent. If one ofBob’s genes is faced with a choice whether to help Maddy, the gene is equally likely to bepadumnal, and necessarily absent from Maddy, or madumnal, with one chance in two ofbeing present in Maddy (presence or absence, of course, referring to identity by recentcommon descent). Therefore, an uninformed gene has one chance in four of being present inMaddy and would favor transfer of the benefit if B >4C.What theoretical predictions can be made about how the conflict would be resolved if therelevant genes are imprinted and thus possess information about their parental origin (Goos &D. Haig / Evolution and Human Behavior 24 (2003) 418–425 419Silverman, 2001; Haig, 2002)? The simple answer is little: genes of both parental originswould favor withholding the benefit from Maddy if B <2C and both sets of genes would favorconferring the benefit if this directly benefited Bob (C < 0). Specific prediction of outcomeswithin the zone of conflict defined by B >2C > 0 require assumptions about the mode of geneaction and the relative power of the contending parties to influence the decision.One possible resolution of a conflict is that one party has the power to dictate the outcome. IfBob’s padumnal genome had the dictatorial role, the benefit would only be conferred if C V 0,whereas, if Bob’s madumnal genome had the dictatorial role, the benefit would be conferredwhenever B >2C. In the first case, Bob’s padumnal genome can be considered to have vetopower as to whether the benefit is conferred, whereas, in the second case, Bob’s madumnalgenome can be considered to be able to present his padumnal genome with a fait accompli(Haig, 1992). Whether a particular dictatorial decision is interpreted as a veto or fait accomplimay simply depend on how the question is framed. Models of gene expression at an imprintedlocus result in a dictatorial outcome when madumnal and padumnal alleles have conflictinginterests over the level of gene expression. At evolutionary equilibrium, whichever allele favorsthe higher amount produces that amount