Are the facial gender and facial age variants of the composite face illusion products of a common mechanism?

When the upper half of one face ('target region') is spatially aligned with the lower half of another ('distractor region'), the two halves appear to fuse together perceptually, changing observers' subjective perception of the target region. This 'composite face illusion' is regarded as a key hallmark of holistic face processing. Importantly, distractor regions bias observers' subjective perception of target regions in systematic, predictable ways. For example, male and female distractor regions make target regions appear masculine and feminine; young and old distractor regions make target regions appear younger and older. In the present study, we first describe a novel psychophysical paradigm that yields precise reliable estimates of these perceptual biases. Next, we use this novel procedure to establish a clear relationship between observers' susceptibility to the age and gender biases induced by the composite face illusion. This relationship is seen in a lab-based sample (N = 100) and is replicated in an independent sample tested online (N = 121). Our findings suggest that age and gender variants of the composite illusion may be different measures of a common structural binding process, with an origin early in the face-processing stream.