This video shows how a mother tries to learn her child not to trust strangers.

But maybe she shouldn’t worry that much? Just like adults, children as young as 3 tend to judge an individual’s character traits, such as trustworthiness and competence, simply by looking at the person’s face, new research shows. And they show remarkable consensus in the judgments they make. Overall, children seemed to be most consistent in judging trustworthiness. This research suggests that children may tend to pay particular attention to the demeanor of a face, more specific whether it is broadly positive or negative.

Also the Guardian wrote about this new research, but takes the implications of the study way too far to my taste.

A little bit of bonus research:
Three-year-olds show greater suspicion of circular arguments than adults

If you don’t have access to the article, do read this press release with my bold and italics (and you’ll see why) :

“If adult-child agreement in face-to-trait inferences emerges gradually across development, one might infer that these inferences require prolonged social experience to reach an adultlike state,” Cogsdill and colleagues write. “If instead young children’s inferences are like those of adults, this would indicate that face-to-trait character inferences are a fundamental social cognitive capacity that emerges early in life.”

To explore these ideas, the researchers had 99 adults and 141 children (ages 3 to 10) evaluate pairs of computer-generated faces that differed on one of three traits: trustworthiness (i.e., mean/nice), dominance (i.e., strong/not strong), and competence (i.e., smart/not smart).

After being shown a pair of faces, participants might be asked, for example, to judge “which one of the people is very nice.” As expected, the adults showed consensus on the traits they attributed to specific faces. And so did the children.

Children ages 3-4 were only slightly less consistent in their assessments than were 7-year-olds . But the older children’s judgments were in as much agreement as adults’, indicating a possible developmental trend.

Overall, children seemed to be most consistent in judging trustworthiness, compared to the other two traits. This suggests that children may tend to pay particular attention to the demeanor of a face — that is, whether it is broadly positive or negative.

Importantly, the findings do not address the question of whether the judgments the children are making are accurate inferences of character. Rather, they simply demonstrate that adults and children are consistent in the traits they attribute to faces, irrespective of the validity of those judgments.

While it is still unclear exactly when the tendency to infer character from faces first emerges, it might be possible to test younger children with the same computer-generated faces to find out.

“If such inferences take root early in development, as the data suggest, even infants might associate faces with trait-consistent behaviors, such as those conveying prosociality,” the researchers note.

Abstract of the research:

Human adults attribute character traits to faces readily and with high consensus. In two experiments investigating the development of face-to-trait inference, adults and children ages 3 through 10 attributed trustworthiness, dominance, and competence to pairs of faces. In Experiment 1, the attributions of 3- to 4-year-olds converged with those of adults, and 5- to 6-year-olds’ attributions were at adult levels of consistency. Children ages 3 and above consistently attributed the basic mean/nice evaluation not only to faces varying in trustworthiness (Experiment 1) but also to faces varying in dominance and competence (Experiment 2). This research suggests that the predisposition to judge others using scant facial information appears in adultlike forms early in childhood and does not require prolonged social experience.