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The nursing hypothesis: An evolutionary account of emotional modulation of the postauricular reflex


  • Preliminary reports of these findings were included in a master's thesis by the first author, a poster at the 50th Annual Conference of the Society for Psychophysiological Research, and a published abstract in the conference proceedings for that meeting. We thank Thomas Schilling, Joe Kennedy, and Sarah Louderman for assistance with the data collection, along with Karen Hebert and Kevin Wise for helpful comments.

Address correspondence to: Dr. Steven Hackley, Department of Psychological Sciences, 210 McAlester Hall, University of Missouri—Columbia, Columbia, MO 65211. E-mail:


The postauricular reflex (PAR) is anomalous because it seems to be potentiated during positive emotions and inhibited during negative states, unlike eyeblink and other components of the startle reflex. Two evolutionary explanations based on simian facial emotion expressions were tested. Reflexes were elicited while 47 young adult volunteers made lip pursing or grimacing poses and viewed neutral, intimidating, or appetitive photos. The PAR was enhanced during appetitive slides, but only as subjects carried out the lip-pursing maneuver. These results support the nursing hypothesis, which assumes that infant mammals instinctively retract their pinnae while nursing in order to comfortably position the head. Appetitive emotions prime the ear-retraction musculature, even in higher primates whose postauricular muscles are vestigial.

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