I'm sure it won't have escaped your notice that over the last 20 years or so the number of different faces and facial expressions given to minifigs has increased considerably, and continues to do so.
Dr Christoph Bartneck, author of the unofficial LEGO minifig catalogues, has published a paper based on responses to an online questionnaire which aimed to understand what emotions the faces of minifig express.
Here's an abstract of the paper:
Toys play an important role in the development of children. LEGO bricks are one of the world's most popular toys and the Minifigure is the centerpiece of every LEGO construction. We investigate and present a summary of the development of the facial expression for all LEGO Minifigures that were released between 1975 and 2010. Our findings are based on several statistical tests that are preformed on data gathered from an online questionnaire.
The results show that the LEGO company started in 1989 to dramatically increase the variety of facial expressions. The two most frequent expressions are happiness and anger and the proportion of happy faces is decreasing over time. Through a k-cluster analysis we identified six types of facial expression: disdain, confidence, concern, fear, happiness, and anger. In addition we tested if the perception of the face changes when the face is presented in the context of a complete Minifigure. The impression of anger, disgust, sadness and surprise were significantly influenced by the presence of context information.
The distinctiveness of the faces was, however, not significantly improved. The variation in skin color did also not change the perception of the Minifigure's emotional expression. We speculate that The children of today will remember LEGO toys not with the same overall positive associations as the current adults do, but may remember the full complexity of faces that act in conflict situations.
I'm not entirely convinced that it was a worthwhile piece of research or that it tells us anything we didn't already know, but if you are interested in scientific analysis it will make interesting reading.
(Minifig images from The Daily Brick)