Expectation Effect

Perception & Judgment

Bruner and Minturn (1955)

Do we see things as they are or as we expect them to be?


Looking at the picture below horizontally, you will see  “A, B, C”; but looking at it vertically, you will now see “12, 13, 14”, turning the “B” into a “13”. Folk psychology (our common understanding of human behavior before we actually study psychology) would have us believe that perception is a clear-cut bottom-up process: external stimuli reach our visual system, which passively registers what is “out there” and transfers this information to higher cognitive functions for further processing, such as identification.

Indeed, even Gestalt Theory described perception as a two-step process involving “perceptual organization” first, and “perceptual identification” second. During the first stage, our mind would simply organize its visual field: differentiate foreground and background, delineate contours, etc. Only after this process is completed would a second stage start, during which our thinking (and especially our memory) helps us recognize the elements of our visual environment. However, as the simple example above illustrates, it seems that this “second” stage can actually alter the first one, thus reversing the process and making perception an active “top-down” exercise in which we select and arrange external stimuli according to certain predispositions, including expectations (but also motivation, emotion, past experience, etc.). It is not that we simply “interpret” what we see based on what we expect: what we expect changes what we actually see. In the example above, expecting a letter or a number makes us see either a “B” as a closed unit, or a “13” composed of two distinct digits. According to Vernon (1955), this phenomenon can be explained by Bartlett’s (1932) Schema Theory as “we perceive certain aspects of the perceptual field in accordance with the schematic category of events to which at the moment they seem to appertain.” Thus, perception comes with biases, as it processes external stimuli within an internal frame of reference.

As defined by Vernon (1955), schemas are: “persistent, deep-rooted, and well-organized classifications of ways of perceiving, thinking, and behaving”. One of their functions is to organize the knowledge stored in memory in a way that is meaningful and relevant. Another one of their functions is to produce expectations. In both cases, schematic processing speeds up cognition, making it more efficient—for instance helping identify ambiguous stimuli, such as a “broken B”. However, schematic processing also lends cognition to error, as expectations can prove to be false assumptions.

Participants & Procedures

24 students from Harvard University (Cambridge, MA, USA).

To test this theory, Bruner and Minturn (1955) presented their subjects with series of capital letters and 2-digit numbers  that included a “broken B” that could be seen as either a “B” or a “13”. Depending on the trial, the broken B was either preceded by individual letters or by individual numbers. The trials were counterbalanced across participants (who were tested one by one), and followed by a control condition with with mixed series of 2 letters and 2 numbers in different orders (so that the broken B could be seen as both a “B” and a “13”). Having introduced the study as an investigation into the speed at which people can recognize letters and numbers, the researchers also gradually increased exposure for each participant until they reached perfect accuracy.  As for the experimental task, the researchers instructed the participants to both tell them and draw what they saw—making it clear that they were to do exactly that: not transcribe what they saw in their own handwriting, but copy it “as is”, including if they perceived an incomplete stimulus.


As can be seen in the tables above, expectations did not only influence what the participants reported seeing, but also what they actually saw and drew. In the “Letter” condition, 16 of the 24 participants drew the ambiguous “broken-B” either fully or partly closed, while only 2 did so in the “Number condition”. This effect decreased, but was still present as exposure increased in duration. This is in line with Schema Theory and suggests that both the cognitive interpretation and the very visual processing of stimuli involve an assimilation to the expectations associated with the category (schema) used to identify them. Indeed, as the researchers concluded, “there is a wide range of phenomena in everyday perception that are precisely of this order: where we see in terms of the properties of objects as there are conceived and fail to ‘notice’ those features that deviate from this conception.”

IA Tip

In line with IB guidelines, we recommend that students only compare two conditions in their experiment and obtain a single measurable result for each participant in each condition. Doing otherwise would complicate inferential statistics without any benefit as far as the IA is concerned.

While the original study asked participants to identify and to draw the letters/numbers presented to them, students should only replicate one of the two tasks. Likewise, while the original study compared how many subjects saw the “broken-B” as a letter or as a number, students could compare how many times each of them does so on a number of trials.


Bruner, J. S., & Minturn, A. L. (1955). Perceptual identification and perceptual organization. Journal of General Psychology, 53, 21–28.