When in infancy does the "fear bias" develop?

Much research has focused on how infants respond to emotional facial expressions. One of the key findings in this area of research is that by 7months of age, but not younger, infants show a bias in processing fearful faces even when compared with other negative and novel facial expressions. A recent study by Heck and colleagues (Journal of Experimental Child Psychology, 2016, Vol. 147, pp. 100-110) challenges this idea by showing that 5-month-olds looked longer at fearful faces than at happy and at neutral faces when dynamic displays (videos) are used. Given that previous work failed to find enhanced attention to fearful faces in 5-month-olds using static displays (photographs), this was taken as evidence that biased attention to fear can be observed earlier when dynamic information is presented. However, we computed an analysis indicating that the overall amount of motion displayed in the videos in Heck and colleagues' study is confounded with emotion such that the greatest amount of motion is evident in the fearful face videos and may have driven infants' looking patterns. We discuss these findings and their limitations in the context of other research using dynamic emotion stimuli. Although these findings do not rule out the possibility that 5-month-olds are sensitive to fear, we stress the need to control for physical differences such as motion before any conclusions regarding the emergence of the fear bias during infancy can be drawn and in order to improve research practice in the field.