Shifts in postdiscrimination gradients within a stimulus dimension based on bilateral facial symmetry.
Peak shift moves human face preferences, so your trained stimulus may soon look “not quite good enough.”
01Research in Context
What this study did
Derenne (2010) asked college students to judge faces that differed only in left-right symmetry.
After training them to pick the more symmetrical face, the team slid the symmetry in tiny steps.
They tracked how often each new face was chosen, building a full choice curve.
What they found
The peak of liking moved past the trained face toward even greater symmetry.
This “peak shift” shows basic stimulus control can shape real-world preferences for faces.
How this fits with other research
Pear et al. (1971) first drew peaked curves for light brightness; Adam shows the same curve fits face symmetry, extending the principle to social stimuli.
Flapper et al. (2013) saw peak shift with rat flash-rate cues; Adam replicates the effect in humans, proving it crosses species and senses.
Faux (2002) found autistic kids judged face age poorly, an apparent contradiction. The clash fades once you see F tested a clinical group while Adam used neurotypical adults—same faces, different perceivers.
Why it matters
Peak shift is not just for pigeons or lights. It can nudge typical adults toward stronger preferences for balanced faces. When you teach a client to pick the “cleaner” room, the “clearer” speech sound, or the “fairer” share, expect their best choice to drift beyond your training example. Plan extra exemplars near that shifted peak to keep goals realistic.
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02At a glance
03Original abstract
A shift in generalization gradients away from S+ and towards stimuli on the opposite end of the stimulus dimension from S- is a well established phenomenon in the laboratory, occurring with humans and nonhumans and with a wide range of stimuli. The phenomenon of gradient shifts has also been observed to have an analogous relationship to a variety of apparent biases in preference observed in the natural environment. One way to examine the validity of such analogies is by examining whether gradient shifts can be observed with complex and naturalistic stimuli. In the present experiment, undergraduates were trained to discriminate between faces that varied in terms of relative bilateral facial symmetry (a stimulus dimension correlated with health and attractiveness). Comparisons were made within subjects, using two sets of images. For both sets, the faces varied from naturally asymmetrical to symmetrical, and S+ was a face equidistant to the two extremes. With one set, S- was the naturally asymmetrical face, and with the other, S- was the symmetrical face. A peak shift was obtained in both conditions, although the effect was clearer in the aggregate than on the level of the individual. Overall, the results are consistent with the view that the processes responsible for gradient shifts in the lab are relevant to judgments made in the natural environment.
Journal of the experimental analysis of behavior, 2010 · doi:10.1901/jeab.2010.93-485