A very interesting study on infant behavior and development by Sabrina S. Chiarella and Diane Poulin-Dubois, at ScienceDirect.com:

The current study examined how 18-month-old infants react to a “stoic” person, that is, someone who displays a neutral facial expression following negative experiences. Infants first watched a series of events during which an actor had an object stolen from her. In one condition, infants then saw the actor display sadness, while she remained neutral in the other condition. Then, all infants interacted with the actor in emotional referencing, instrumental helping, empathic helping, and imitation tasks. Results revealed that during the exposure phase, infants in both groups looked an equal amount of time at the scene and engaged in similar levels of hypothesis testing. However, infants in the sad group expressed more concern toward the actor than those in the neutral group. No differences were found between the two groups on the interactive tasks. This conservative test of selective learning and altruism shows that, at 18 months, infants are sensitive to the valence of emotional expressions following negative events but also consider an actor’s neutral expression just as appropriate as a sad expression following a negative experience. These findings represent an important contribution to research on the emergence of selective trust during infancy.