In discussing the various strategies by which people may be able to pull the wool over their own eyes, Von Hippel & Trivers (VH&T) acknowledge implicitly that the default mode is self-knowing rather than self-deception. That is to say, other things being equal, people have a remarkable degree of insight into their own behavior and have to take active steps to avoid this if and when it does not suit them. There are good reasons why it should be so. Self-awareness brings substantial benefits. Not least, as I have argued, self-awareness underlies people's ability to develop a theory of human mind. Just to the extent that people know from the inside how they themselves truly felt and thought in a given situation, they can imagine what the same situation will be like for someone else – and so will be able to simulate how others are likely to behave. It is precisely the evolved capacity for conscious insight – veridical insight – that has allowed human beings to become what I have called “natural psychologists,” with an unparalleled ability to predict and manipulate the behavior of other members of their species (Humphrey Reference Humphrey1978).

However, if this is so, it clearly makes difficulties for VH&T's – otherwise convincing – theory that there are situations where it is better for an individual to be blind to the psychological reality; or, at any rate, it means VH&T are seeing only half of the picture. Given that in general self-knowing brings with it a greater understanding of others, the corollary has to be that self-deception brings about lesser understanding. In particular, if and when someone fails to recognize that he himself has behaved in a mendacious way, he is less likely to recognize the mendacity of others. Thus, although it may well be the case that people who deceive themselves when they cheat are less likely to be caught, such people are also more likely to be duped by others. As I remarked in my book Consciousness Regained, “It takes a thief to catch a thief and an intimate of his own consciousness to catch the intimations of consciousness in others” (Humphrey Reference Humphrey1983, p. 63).

Evidence that this is in fact how things play out has been provided by Surbey (Reference Surbey, Crawford and Salmon2004). In a study with Rankin, Surbey gave subjects a self-deception questionnaire and also a personality test to assess Machiavellianism – the ability to get the better of others through psychological manipulation. It turned out that people with strong Machiavellian tendencies got low scores on the self-deception test. “They're more consciously aware of selfish motivations than others,” Surbey is quoted as saying, “and they're projecting their selfish motivations on others.” (Motluk Reference Motluk2001).

VH&T's theory would predict that self-deception – having little downside – should be a universal human trait. However, Surbey and Rankin found big individual differences, with some people being highly self-deceptive and others hardly at all. But this is just what we should expect if self-deception does have this downside, that is, if it makes people better thieves but poorer detectives. For this means there will have been balancing selection in the course of human evolution. Assuming that self-deceivers will have won out when few people suspected deceit, but suspicion will have won out when most people were self-deceivers, selection will have resulted in a mix of strategies in the human population – a classic balance between doves and hawks.