Across many psychological domains there is evidence of negativity bias: the greater subjective potency of negative events when compared with positive events of the same objective magnitude. Here I propose a general evolutionary explanation for the phenomenon: the concave fitness-state (CFS) hypothesis. The CFS hypothesis proposes, with evidence from feeding, drinking and economic domains, that various motivational, emotional and cognitive states – through which stimuli activate responses – have a concave downwards (diminishing returns) relationship with fitness. Where this is the case it follows that a reduction in state, due to a negative stimulus, reduces fitness more than a positive stimulus of equal objective magnitude increases it, producing – the hypothesis consequently predicts – a negativity bias due to the difference in subjective stimulus potency. In discussing other approaches to understanding the phenomenon I critique the proposal that negativity bias can be explained as an adaptive response to the particular importance and urgency of dealing with threat, by arguing that: (1) where negative stimuli interpretable as threat, and contrasting positive stimuli, cannot be measured in a commensurate manner they cannot be validly tested for negativity bias; and (2) since threat stimuli and positive stimuli generally impact different states a greater potency for threat stimuli should generally be interpreted in terms of motivational competition rather than negativity bias. I suggest two ways of circumventing the problem of incommensurate stimuli when studying stimulus bias. The first is to use the loss aversion paradigm: rating the value of the same stimulus when presented as either a gain or a loss in relation to a reference value. Second, understanding the relative subjective potencies of positive and negative stimuli across a range of objective stimulus magnitudes, even when incommensurate, can be achieved experimentally by finding pairs of positive and negative stimuli which, though measured on different scales of magnitude, are equipotent. That is, they have equal and opposite effects on fitness, well-being or stimulus evaluation. These stimulus pairs constitute a potency equivalence function, which describes the shape of the relationship between equipotent positive and negative stimulus magnitudes.

在许多心理领域中，都有证据表明存在消极偏见：与相同客观大小的积极事件相比，消极事件的主观效力更大。在此，我对这种现象提出一个一般的进化解释：凹适态（CFS）假设。CFS假说提出了从饮食，饮食和经济领域的证据，各种动机，情绪和认知状态（刺激通过这些状态激活反应）与健身之间呈凹形向下（收益递减）关系。在这种情况下，随之而来的是，由于负面刺激而导致的状态减少会降低适应度，而与相等客观大小的正面刺激会增加适应度，从而（假设因此预测）由于主观刺激的差异而产生负偏倚效力。在讨论理解现象的其他方法时，我批评以下观点：可以将消极偏见解释为对应对威胁的特殊重要性和紧迫性的适应性反应，其论点是：（1）负面刺激可解释为威胁，和相反的积极刺激，无法以相称的方式进行衡量，也无法有效地测试其消极偏差；（2）由于威胁刺激和积极刺激通常会影响不同的国家，威胁刺激的更大效力通常应从动机竞争而非消极偏见的角度来解释。在研究刺激偏差时，我提出了两种避免不当刺激问题的方法。第一种是使用损失规避范式：将相同刺激的值以相对于参考值的收益或损失来表示。其次，即使在不相称的情况下，也要了解在一系列客观刺激幅度内，正面和负面刺激的相对主观效力，等价的。也就是说，它们对健身，幸福感或刺激性评估具有同等和相反的作用。这些刺激对构成了一个效当量函数，该函数描述了等电位正刺激量和负刺激量之间关系的形状。