Babies at 15 months of age make snap judgments when a stranger shows anger and they don't forget quickly, child psychologists find.

Babies closely watch how people interact. Like adults, they can quickly form negative impressions.

Now researchers at the University of Washington have tested more than 250 boys and girls aged 15 months to see how they react when exposed to an unfamiliar adult's anger.

The researchers say by 12 months of age, babies are very responsive to other people's emotions and they use that emotional information to regulate their behaviour and generalize. 

"You never get a second chance to make a first impression and it seems to be that babies follow that old saying as well," said Betty Repacholi, an associate professor at the University of Washington.

In the March issue of Developmental Psychology, Repacholi and her co-authors reported babies generalize an adult's angry behaviour to a new situation.

"It's as if babies knew that the best strategy was to give this anger prone person exactly what she wanted so that she would not get angry at them," she said.

Baby boy

Babies clearly tune in to the tone of voice of adults, say researchers. (Shutterstock)

In the experiment, each baby sat on their mother or father's lap as an experimenter showed how to play with a series of toys. Given the complexity of the tasks, the researchers chose to study 15-month-olds. 

During each trial, a second researcher or "emoter" reacted either neutrally or negatively, such as saying, "That's aggravating," in a stern voice.

Based on previous research, the study's authors chose emotional words that were too difficult for 15-month-olds to understand. It's assumed they would respond to the angry tone of voice and angry facial expression. The babies weren't able to understand the words but they clearly tuned into tone of voice.

At 15 months of age, babies aren't good at sharing toys, Repacholi said. So it's all the more impressive that when babies were handed an attractive toy, they were more likely to hand it over if the researcher had a history of being angry.

Repacholi called the baby's appeasement "a really sensible and very sophisticated response."

"Their strategy was better safe than sorry."

OK to scold in front of baby

First impressions matter in children, says Patricia Brosseau-Liard, an assistant professor in psychology at the University of Ottawa. Brosseau-Liard, who was not involved in this study, has done studies on how children evaluate people's credibility.

"A lot of this research now shows that children do have some understandings about their social world sometimes much earlier than we previously thought," Brosseau-Liard said.

She said it's important for parents and educators to know how attentive children are to the interactions around them.

"Here interestingly it is not just for example the emotions that others display towards them, but also emotions that other people display towards each other," Brosseau-Liard said. 

People should also keep the findings in perspective, Repacholi said. "I don't want parents to feel guilty because their baby might have happened to hear them scolding an older sibling."

A baby will take into account the range of emotions parents express.

Repacholi hopes to look at how the impressions endure over time and how children generalize the information to other people. So far, the longest they've tested how long the impressions last is five minutes. She suspects it could last for a couple of days, if no new information is provided. 

Demonstration of an earlier baby emotion experiment