Minimizing the costs that others impose upon oneself and upon those in whom one has a fitness stake, such as kin and allies, is a key adaptive problem for many organisms. Our ancestors regularly faced such adaptive problems (including homicide, bodily harm, theft, mate poaching, cuckoldry, reputational damage, sexual aggression, and the infliction of these costs on one's offspring, mates, coalition partners, or friends). One solution to this problem is to impose retaliatory costs on an aggressor so that the aggressor and other observers will lower their estimates of the net benefits to be gained from exploiting the retaliator in the future. We posit that humans have an evolved cognitive system that implements this strategy -deterrence -which we conceptualize as a revenge system. The revenge system produces a second adaptive problem: losing downstream gains from the individual on whom retaliatory costs have been imposed. We posit, consequently, a subsidiary computational system designed to restore particular relationships after cost-imposing interactions by inhibiting revenge and motivating behaviors that signal benevolence for the harmdoer. The operation of these systems depends on estimating the risk of future exploitation by the harmdoer and the expected future value of the relationship with the harmdoer. We review empirical evidence regarding the operation of these systems, discuss the causes of cultural and individual differences in their outputs, and sketch their computational architecture.

Objective There is increased interest in measuring peripheral oxytocin levels to better understand the role of this peptide in mammalian behavior, physiology, and disease. The purpose of this study was to compare methods for plasma oxytocin measurement using a commercially available enzyme immunoassay (EIA) and radioimmunoassay (RIA), to evaluate the need for sample extraction, and to assess the immunospecificity of the assays. Methods Oxytocin was measured in extracted and unextracted human plasma samples (n = 39). Oxytocin and its degradation products were separated by high performance liquid chromatography and gel filtration chromatography and then assayed by EIA or RIA to identify oxytocin immunoreactive peaks. Results Without extraction, plasma measured by EIA was more than 100-fold higher than in extracted plasma, and the correlation between oxytocin levels in extracted and unextracted plasma was minimal (Spearman’s rho = −0.10, p = 0.54). Using the RIA, most samples (> 90%) were below the level of detection with or without extraction. Following chromatographic fractionation of sample extracts, multiple immunoreactive products were found to be present in addition to oxytocin, which casts doubts on the specificity of the assays. Conclusions Changes in oxytocin levels have been reported in social and behavioral challenge studies. This study indicates that sample extraction is necessary to obtain valid assay results. Changes in oxytocin degradation products are likely to contribute to the previously observed responses in circulating oxytocin levels to behavioral and social challenge. There is a critical need for validated and reliable methods to measure oxytocin in biological samples.

In two studies, the authors sought to identify the mathematical function underlying the temporal course of forgiveness. A logarithmic model outperformed linear, exponential, power, hyperbolic, and exponential-power models. The logarithmic function implies a psychological process yielding diminishing returns, corresponds to the Weber-Fechner law, and is functionally similar to the power law underlying the psychophysical function (Stevens, 1971) and the forgetting function (Wixted & Ebbesen, 1997). By 3 months after their transgressions, the typical participant's forgiveness had increased by two log-odds units. Individual differences in rates of change were correlated with robust predictors of forgiveness. Consistent with evolutionary theorizing (McCullough, 2008), Study 2 revealed that forgiveness was uniquely associated with participants' perceptions that their relationships with their offenders retained value.

Across and within societies, people vary in their propensities towards exploitative and retaliatory defection in potentially cooperative interaction. We hypothesized that this variation reflects adaptive responses to variation in cues during childhood that life will be harsh, unstable and short-cues that probabilistically indicate that it is in one's fitness interests to exploit co-operators and to retaliate quickly against defectors. Here, we show that childhood exposure to family neglect, conflict and violence, and to neighbourhood crime, were positively associated for men (but not women) with exploitation of an interaction partner and retaliatory defection after that partner began to defect. The associations between childhood environment and both forms of defection for men appeared to be mediated by participants' endorsement of a 'code of honour'. These results suggest that individual differences in mutual benefit cooperation are not merely due to genetic noise, random developmental variation or the operation of domain-general cultural learning mechanisms, but rather, might reflect the adaptive calibration of social strategies to local social-ecological conditions.