<p>According to the heuristics and biases approach, the representativeness heuristic (RH) is one of the heuristics available for assessing subjective probabilities (A. Tversky & D. Kahneman, 1974). A subjective probability assessed by the RH is determined by how representative the target object is of the target category. Several aspects of the RH are argued to cause systematic biases, for example: (<i>i</i>) When the RH is used, the category is represented by one single prototypical exemplar. This feature is argued to cause biases such as misperception of chance and insensitivity to sample size. (<i>ii</i>) The RH assesses the inverse rather than the conditional probability. This feature is argued to cause biases such as the conjunction fallacy and base-rate neglect.</p><p>The present thesis focuses on the cognitive aspects of the RH. Three studies were conducted. Overall, data indicated that the RH does not play a major role when subjective probabilities are assessed. Study I indicated that subjective probabilities are not typically determined by how representative the target object is of the target category. Study II indicated that the category is not represented by one single prototypical exemplar when subjective probabilities are assessed. Study III indicated that conjunction fallacies are not caused by the RH. </p><p>The results presented in Studies I-III cast serious doubts on the claim that subjective probabilities are routinely assessed using the RH. Rather, Studies I-II suggested that subjective probabilities are based on exemplar memory and Study III suggested that the conjunction fallacy is caused by people combining component probabilities in a an inappropriate way. In the General Discussion, it is suggested that people use a weighted average rule when combining component probabilities into conjunction probabilities. A simulation showing the ecological relevance of the weighted average rule is presented.</p>

According to the heuristics and biases approach, the representativeness heuristic (RH) is one of the heuristics available for assessing subjective probabilities (A. Tversky &amp; D. Kahneman, 1974). A subjective probability assessed by the RH is determined by how representative the target object is of the target category. Several aspects of the RH are argued to cause systematic biases, for example: (i) When the RH is used, the category is represented by one single prototypical exemplar. This feature is argued to cause biases such as misperception of chance and insensitivity to sample size. (ii) The RH assesses the inverse rather than the conditional probability. This feature is argued to cause biases such as the conjunction fallacy and base-rate neglect. The present thesis focuses on the cognitive aspects of the RH. Three studies were conducted. Overall, data indicated that the RH does not play a major role when subjective probabilities are assessed. Study I indicated that subjective probabilities are not typically determined by how representative the target object is of the target category. Study II indicated that the category is not represented by one single prototypical exemplar when subjective probabilities are assessed. Study III indicated that conjunction fallacies are not caused by the RH. The results presented in Studies I-III cast serious doubts on the claim that subjective probabilities are routinely assessed using the RH. Rather, Studies I-II suggested that subjective probabilities are based on exemplar memory and Study III suggested that the conjunction fallacy is caused by people combining component probabilities in a an inappropriate way. In the General Discussion, it is suggested that people use a weighted average rule when combining component probabilities into conjunction probabilities. A simulation showing the ecological relevance of the weighted average rule is presented.

Heuristic estimates of probabilities may be an obstacle to decision making within High Reliability Organizations. Accident reports have found that two from each other separate phenomenon, Blame Culture and Type 1 processing constitutes a particularily serious threat to decision making. The present study (N = 70) investigated if a perceived risk of negative feedback and cognitive load would lead to more heuristic estimates on the Conjunction Fallacy. Three experiment conditions were included in the study: Negative feedback, cognitive load and control. The results were non-significant for both negative feedback and cognitive load. Furthermore, the estimated negative affect was higher when violations to the Conjunction Rule was made. Previous studies showing that high scores on the Cognitive Reflection Test (CRT) indicate less sensitivity to conjunction fallacies, were replicated. The present study concluded that the CRT may be a strong predictor of the Conjunction Fallacy.

Previous research on human judgment and decision making has demonstrated systematic and predictable biases of judgment in experimental settings. One example of this is the tendency to intuitively violate the conjunction rule - a simple rule of probability. This was well illustrated in the famous Linda-problem. (Tversky &amp; Kahneman, 1983). According to the dual-process theory of reasoning, (Kahneman, 2011) reasoning fallacies such as the conjunction fallacy occurs when people fail to use analytic reasoning and instead overly rely on their intuition. The dual process theory proposes that cognitive processes underlying our intuitive impulses and our conscious reasoning constitutes two different modes in the mind –system 1 and system 2- and that the intuitive system 1 are not able to compute probabilities. Furthermore, it is assumed that processes that are labeled system 1 are fast whereas system 2 are thought to be slow. We tested these time course assumptions of dual process theory in a within-subject design by comparing response time latencies between conjunction fallacy judgments and accurate probability judgments. The results showed that inducing accurate responding did not result in delayed response latency. This indicates that making accurate probability judgments does not require more processing time which goes against what would be expected by the dual-process framework. / Tidigare forskning om mänskligt beslutsfattande och bedömningar har i experiment påvisat systematiska och förutsägbara bias. Ett exempel på detta är tendensen att intuitivt gå emot konjuktionsregeln- en enkel regel gällande sannolikhet. Detta illustrerades väl i det berömda Linda- problemet (Tversky &amp; Kahneman, 1983). Enligt två-systemsteorin (Kahneman, 2011) om problemlösning, uppstår bedömningsfel såsom konjuktionsfelet när människor inte använder sig av analytiskt tänkande och istället förlitar sig för mycket på sin intuition. Två-systemsteorin menar att de kognitiva processer som ligger till grund för våra intuitiva impulser och vårt medvetna resonerande utgör två olika aktörer i vårt tänkande- system 1 och system 2- och att det intuitiva system 1 inte kan beräkna sannolikheter. Processer som betecknas som tillhörande system 1 är enligt två-systemsteorin snabba medan system 2 föreställs vara ett långsamt system. I denna studie testades antagandet om tidsåtgång för de två systemen i en inomgruppsdesign genom att jämföra responstider mellan bedömningar där konjunktionsfelet begåtts och bedömningar där det inte begåtts. Resultaten visar att korrekta sannolikhetsbedömningar inte resulterar i långsammare responstider. Detta indikerar att det inte tycks vara mer tidskrävande att göra riktiga sannolikhetsbedömningar, vilket motsäger två-systemsteorins antagande om snabbt och långsamt processande.