Size matters to babies, too, a study suggests

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By Carolyn Y. Johnson
Globe Staff / January 28, 2011

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Long before they learn about popular kids or bullies — much less how to walk or talk — infants already understand social hierarchies and the fact that size is a key factor in determining who will yield when paths cross, according to a new study by Harvard researchers.

Far from being something children learn on the playground, from pop culture, or in lessons from parents, the importance of size in social hierarchies may be innate or develop very early in life, the research suggests.

The study published yesterday by the journal Science describes an experiment in which babies watched animated cartoons in which big and small blocks bound toward one another, bump against each other, and bow deferentially.

If the big block bowed to the small one, infants as young as 10 months old stared for a long time, indicating to researchers that this bit of social choreography defied their expectations. The new work adds to a growing body of research revealing the powerful social tool kit that babies use to make sense of the world and suggests that they understand some basic rules of social interaction before they have had to use them.

“In a sense, one of the most important tasks any infant has to solve is to figure out, what is the structure of the social world,’’ said Lotte Thomsen, a postdoctoral researcher at Harvard University and assistant professor of psychology at the University of Copenhagen, who led the study. “Who is friend and foe, who is superior, who is a peer.’’

Research into how people first perceive social hierarchies is more than just a fascinating question about early childhood. Our ability to navigate complex social situations is crucial for success in life, and also shapes our behavior, attitudes, and opinions.

Preschoolers, researchers have found, calibrate their social interactions with a playmate depending on whether the other child is of higher or lower social rank. There has been growing concern about the role social hierarchy plays in adolescence, because of bullying in schools. As for adults, research shows that people who say they prefer an abstract symbol that is a hierarchical pyramid — instead of one that shows a more egalitarian arrangement of overlapping circles — are more likely to be racist, ethnocentric, or tolerant of the persecution of others.

In the animal kingdom and across human culture, size is an important factor in establishing social hierarchies, from an animal that puffs itself up to show its dominance to kings or popes who wear big hats or sit up high.

“From my evolutionary perspective, this is an aspect of human social organization that is fundamentally important. And if it’s fundamentally important, then very young children should have some predilection to be tuned in to these cues from very early on,’’ said Patricia Hawley, an associate professor of psychology at the University of Kansas who was not involved in the study. She praised the study for the way researchers systematically ruled out other possible explanations for the babies’ behavior.

Because infants who are still learning their first words cannot describe their thoughts, Thomsen, working with Harvard psychology professor Susan Carey, used a “looking time’’ experiment. Such experiments are well established in developmental psychology, taking advantage of the fact that even babies can formulate expectations about situations. If a scenario runs counter to that expectation, they will stare at it for a longer time.

In the experiments, babies from 8 to 16 months old sat on a parent’s lap and watched animations. A big blue block with an eye and mouth bounced from left to right. Then, a smaller green block bounded in the opposite direction. Researchers then showed the two blocks in conflict: Both started at opposite corners, and when they met in the center, they bumped into each other and reversed direction.

Two scenarios followed. In one, the blocks bounded toward each other and bumped in to each other, then the small green block bowed and moved out of the way so that the big blue block could continue on its way. In the second scenario, the large blue block prostrated itself in front of the small green one.

Researchers froze the screen after each scenario and found that babies consistently stared longer at the unexpected scenario than the expected one — for 20 seconds on average, instead of 12. The difference was noted in babies 10 months old and older, but not in the 8-month-old babies.

Such experiments may sound simple, but researchers tested 144 babies and designed a number of experiments to eliminate other explanations. They determined that infants were not staring longer at the animation because when the bigger block fell there was more mass in motion, or because babies expected that small creatures were more likely to fall down.

These experimental techniques have fostered burgeoning interest in exploring the social abilities of babies. In 2007, J. Kiley Hamlin, now an assistant professor of psychology at the University of British Columbia, showed that infants as young as 6 months old prefer characters that are helpful over ones that hinder. Her lab is working on other social questions as well, such as how babies make character judgments about who deserves what.

For Thomsen, these questions about social hierarchies extend into school and even into adulthood. In her research, she is finding that preschoolers’ preference for overlapping icons, rather than ones that suggest hierarchy, predicts how well they share when given stickers and how secure they feel about friendships.

“The more we understand about how social hierarchy and dominance hierarchies work and how we recognize them and how we manipulate them, that should ultimately also make us able to intervene and shape them in ways that are less harmful,’’ Thomsen said.

Carolyn Y. Johnson can be reached at