Evolutionary Psychology: New Perspectives on Cognition and Motivation

Supplementary Data

• Download the Supplemental Figures 1 through 4 as a PDF (also reproduced below).

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  Supplemental Figure 1. A Wason selection task with a conditional rule that is indicative (describes a relationship) and has familiar content (it links a disease to a symptom). In a Wason selection task, there is always a rule of the form If P then Q, and four cards showing the values P, not-P, Q, and not-Q (respectively) on the side that the subject can see. By first order logic, only the combination of P and not-Q can violate this rule, so the correct answer is to check the P card (to see if it has a not-Q on the back), the not-Q card (to see if it has a P on the back), and no others. Few subjects answer correctly, however, when the conditional rule is descriptive (indicative), even when its content is familiar. For example, only 26% of subjects answered the above problem correctly (by selecting "has Ebbinghaus disease" and "is not forgetful"). Most choose either P alone, or P&Q. (The red italicized Ps and Qs are not in problems given to subjects.)

  

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  Supplemental Figure 2. Wason selection task with a social contract rule. In response to the social contract problem shown in panel A, 76% of subjects chose P & not-Q ("borrowed car" and "did not fill up the tank with gas")—the cards that represent potential cheaters. Yet only 26% chose this (logically correct) answer in response to the descriptive rule in Supplemental Figure 1. Although this social contract rule involves familiar items, unfamiliar social contracts elicit the same high performance. Panel B shows how the mind represents this social contract. The cheater detection mechanism leads subjects to choose the benefit accepted card and the requirement not satisfied card, whether this answer is logically correct or not—see See Supplemental Figure 3.

  

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  Supplemental Figure 3. The same social contract can be expressed in different linguistic formats. Blue indicates how social contract algorithms represent the rule; red indicates how the rule would be represented by content-free logical procedures. According to inferential rules specialized for social exchange "If you take the benefit, then you are obligated to satisfy the requirement" implies "If you satisfy the requirement, then you are entitled to take the benefit". (This inference is not licensed by first-order logic.) The cheater detection mechanism leads subjects to choose the benefit accepted card and the requirement not satisfied card, regardless of how the rule is expressed. When the benefit is in the P clause (standard format), these two cards correspond to P and not-Q—a logically correct response. When the benefit is in the Q clause (switched format), they correspond to Q and not-P—a logically incorrect response. Experiments with switched social contracts have shown that social contracts activate reasoning procedures that look for cheaters, even when this response violates first-order logic. Note that a logically correct response to a switched social contract—where P = requirement satisfied and not-Q = benefit not accepted—would fail to detect cheaters.

  

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  Supplemental Figure 4. Parametric study of social contract reasoning. In all conditions, subjects were asked to look for violations of the same social contract rule. What varied was whether potential violators could benefit by violating the rule (B), whether their violating the rule was by intention or by mistake (I), and whether the situation provided them with the ability to easily violate it (A). When all three factors were present (BIA), performance was highest. It dropped significantly when only two factors were present (BI, BA, IA), and again when only one factor was present (B, I, A)—to the same low level found when none of these factors were present. The results show that violations of social contracts are detected when they are likely to reveal potential cheaters—individuals with a disposition to illicitly take the benefit regulated by the rule. The mechanism is not designed to detect innocent mistakes, especially ones that do not benefit the violator (see Cosmides, Barrett, & Tooby, 2010).