Though Brennan's formalization was introduced in the 1980s, it remains relevant in recent work. Mo et al. refined the idea by noting that the population variance of the feature should be taken into account. For example, the distance between the eyes varies less than other features such as the size of the nose. Thus even a small variation in the eye spacing is unusual and should be exaggerated, whereas a correspondingly small change in the nose size relative to the mean would not be unusual enough to be worthy of exaggeration.