Previous research has shown that people exhibit a sample size bias when judging the average of a set of stimuli on a single dimension. The more stimuli there are in the set, the greater people judge the average to be. This effect has been demonstrated reliably for judgments of the average likelihood that groups of people will experience negative, positive, and neutral events (Price, 2001;Price, Smith, & Lench, 2006) and also for estimates of the mean of sets of numbers (Smith & Price, 2010). The present research focuses on whether this effect is observed for judgments of average on a perceptual dimension. In 5 experiments we show that people's judgments of the average size of the squares in a set increase as the number of squares in the set increases. This effect occurs regardless of whether the squares in each set are presented simultaneously or sequentially; whether the squares in each set are different sizes or all the same size; and whether the response is a rating of size, an estimate of area, or a comparative judgment. These results are consistent with a priming account of the sample size bias, in which the sample size activates a representation of magnitude that directly biases the judgment of average.

Across a wide variety of situations, exposure to anchors has been shown to bias people's estimates. What is not known, however, is whether externally provided anchors influence the confidence that people have in their estimates. Our studies had two goals. First, we tested whether exposure to anchors influenced people's subjective confidence levels (Studies 1 and 2). These studies revealed that people who made estimates after making comparisons with externally provided anchors tended to be more confident in their estimates than people who did not see anchors. The second goal was to test two explanations as to why anchors increase people's confidence. In Study 3, we tested the explanation that anchors increase confidence because participants thought the anchors provided useful information. In Study 4, we tested the explanation that exposure to anchors causes people to consider a narrower range of plausible values as compared to when not exposed to anchors. Support was found only for the explanation that comparisons with anchors increase confidence because people who are exposed to anchors consider a narrower range of plausible values. Taken together, these studies reveal the powerful influence anchors can have—they not only bias estimates, but also increase people's confidence in their biased estimates. Copyright © 2016 John Wiley & Sons, Ltd.