From an evolutionary standpoint, a default presumption is that true beliefs are adaptive and misbeliefs maladaptive. But if humans are biologically engineered to appraise the world accurately and to form true beliefs, how are we to explain the routine exceptions to this rule? How can we account for mistaken beliefs, bizarre delusions, and instances of self-deception? We explore this question in some detail. We begin by articulating a distinction between two general types of misbelief: those resulting from a breakdown in the normal functioning of the belief formation system (e.g., delusions) and those arising in the normal course of that system's operations (e.g., beliefs based on incomplete or inaccurate information). The former are instances of biological dysfunction or pathology, reflecting “culpable” limitations of evolutionary design. Although the latter category includes undesirable (but tolerable) by-products of “forgivably” limited design, our quarry is a contentious subclass of this category: misbeliefs best conceived as design features. Such misbeliefs, unlike occasional lucky falsehoods, would have been systematically adaptive in the evolutionary past. Such misbeliefs, furthermore, would not be reducible to judicious – but doxastically1 noncommittal – action policies. Finally, such misbeliefs would have been adaptive in themselves, constituting more than mere by-products of adaptively biased misbelief-producing systems. We explore a range of potential candidates for evolved misbelief, and conclude that, of those surveyed, only positive illusions meet our criteria.

Positive and negative misbeliefs both may have evolved to serve important adaptive functions. Here, we focus on the role of negative misbeliefs in promoting adaptive outcomes within the contexts of romantic relationships and intergroup interactions. Believing bad things can paradoxically encourage romantic fidelity, personal safety, competitive success, and group solidarity, among other positive outcomes.

The radical evolutionary step that divides human decision-making from that of nonhumans is the ability to excite the reward process for its own sake, in imagination. Combined with hyperbolic over-valuation of the present, this ability is a potential threat to both the individual's long term survival and the natural selection of high intelligence. Human belief is intrinsically “unfounded” or under-founded, which may or may not be adaptive.

Naive physics beliefs can be systematically mistaken. They provide a useful test-bed because they are common, and also because their existence must rely on some adaptive advantage, within a given context. In the second part of the commentary we also ask questions about when a whole family of misbeliefs should be considered together as a single phenomenon.

A large amount of research in cognitive psychology is focused on memory distortions, understood as deviations from various (largely implicit) standards. Many alleged distortions actually suggest a highly functional system that balances the cost of acquiring new information with the benefit of relevant, contextually appropriate decision-making. In this sense many memories may be examples of functionally adaptive misbelief.

Most people hold overly (though not excessively) positive self-views of themselves, their ability to shape environmental events, and their future. These positive illusions are generally (though not always) beneficial, promoting achievement, psychological adjustment, and physical well-being. Social processes conspire to produce these illusions, suggesting that affiliation patterns may have evolved to nurture and sustain them.

McKay & Dennett (M&D) observe that beliefs need not be true in order to evolve. We connect this insight with Schelling's work on cooperative commitment to suggest that some beliefs – ideologies – are best approached as social goals. We explain why a social-interactive perspective is important to explaining the dynamics of belief formation and revision among situated partners.

Although it makes some progress, McKay & Dennett's (M&D's) proposal is limited because (1) the argument for adaptive misbelief is not new, (2) arguments overextend the evidence provided, and (3) the alleged sufficient conditions are not as prohibitive as suggested. We offer alternative perspectives and evidence, including individual differences research, indicating that adaptive misbeliefs are likely much more widespread than implied.

Beliefs may be true or false, and grounded or ungrounded. McKay & Dennett (M&D) treat these properties of belief as independent. What, then, do they mean by misbelief? They state that misbeliefs are “simply false beliefs.” So would they consider a very well-grounded belief that is false a misbelief? And why can't beliefs that are very poorly grounded be considered delusions, even when they are true?

Focusing on the individual's internal cognitive architecture, McKay & Dennett (M&D) provide an incomplete analysis because they neglect the crucial role played by the external environment in producing misbeliefs and determining whether those misbeliefs are adaptive. In some environments, positive illusions are not adaptive. Further, misbeliefs often arise because the environment commonly fails to provide crucial information needed to form accurate judgments.

In this commentary, I question the idea that positive illusions are evolved misbeliefs on the grounds that positive illusions are often maladaptive, are not universal, and may be by-products of existing mechanisms. Further, because different beliefs are adaptive in different situations and cultures, it makes sense to build in a readiness to form beliefs rather than the beliefs themselves.

McKay & Dennett (M&D) argue that positive illusions are a plausible candidate for a class of evolutionarily “selected for” misbeliefs. I argue (Flanagan 1991; 2007) that the class of alleged positive illusions is a hodge-podge, and that some of its members are best understood as positive attitudes, hopes, and the like, not as beliefs at all.

This commentary highlights the distinction between belief and pragmatic acceptance, and asks whether the positive illusions discussed in section 13 of the target article may be judicious pragmatic acceptances rather than adaptive misbeliefs. I discuss the characteristics of pragmatic acceptance and make suggestions about how to determine whether positive illusions are attitudes of this type.

We propose another positive illusion – overconfidence in the generalisability of one's theory – that fits with McKay & Dennett's (M&D's) criteria for adaptive misbeliefs. This illusion is pervasive in adult reasoning but we focus on its prevalence in children's developing theories. It is a strongly held conviction arising from normal functioning of the doxastic system that confers adaptive advantage on the individual.

We argue that many evolved biases produced through selective forces described by error management theory are likely to entail misbeliefs. We illustrate our argument with the male sexual overperception bias. A misbelief could create motivational impetus for courtship, overcome the inhibiting effects of anxiety about rejection, and in some cases transform an initially sexually uninterested woman into an interested one.

I take up the challenge of why false beliefs are better than “cautious action policies” (target article, sect. 9) in navigating adaptive problems with asymmetric errors. I then suggest that there are interactions between supernatural beliefs, self-deception, and positive illusions, rendering elements of all such misbeliefs adaptive. Finally, I argue that supernatural beliefs cannot be rejected as adaptive simply because recent experiments are inconclusive. The great costs of religion betray its even greater adaptive benefits – we just have not yet nailed down exactly what they are.

Rather than being a genuine adaptation, “positive illusions” are examples of doxastically uncommitted policies implemented at both the individual and societal levels. Even when they are genuine misbeliefs, most positive illusions are not evolved but ephemeral – a phenomenon limited to a particular social and economic moment. They are essentially a consumer response to messages from the pop-psychology industry in the recently terminated era of easy credit.

McKay & Dennett (M&D) have done an admirable job of distinguishing among various forms of misbelief and evaluating the idea that they stem from evolved mental mechanisms. We argue that a complete account of misbeliefs must attend to the role that others play in creating and maintaining positive illusions.

We question a central premise upon which the target article is based. Namely, we point out that the evidence for “positive illusions” is in fact quite mixed. As such, the question of whether positive illusions are adaptive from an evolutionary standpoint may be premature in light of the fact that their very existence may be an illusion.

In their pursuit of adaptively biased misbelief-making systems, McKay & Dennett (M&D) describe a putative doxastic shear-pin system which enables misbeliefs to form in situations of extreme psychological stress. Rather than discussing their argument, I consider how this shear-pin system might combine with both pathological belief-making (“culpable” breakdowns caused by neuropathy) and normal belief-making to explain a spectrum of delusions.