Social hierarchy is a fundamental structure for social interactions and has a great influence on the behavior and physiology in several animal species, including humans. Even in preschool children, we can easily observe hierarchical relationship forming early in life and gradually becoming stable with age. Studies of social interactions in young children have indicated that children with more aggressive personalities are generally more likely to win fights and acquire dominant positions. However, in addition to be coercive, learning and adoption of social strategies are also important for children to achieve higher status. Why can some children effectively learn and flexibly use social strategies while other children exposed to the same school instruction cannot? In this study, our data suggested that, in both young mice and children, there is a positive correlation between dominance hierarchy and memory abilities, which may provide us some insights related to the formation of social hierarchy in preschool children.
The motivation behind this project was initiated during a visit at an elementary school. I have been working on mouse social hierarchy for a few years, but when I noticed that there is an obvious hierarchical relationship among children in a class, I started to wonder why we only investigate the social hierarchy in adult male mice. I believed that it was because female or young mice rarely displayed aggressive behaviors. Therefore, back at the lab, I decided to use the tube test assay, in which one mouse forces its opponent backward out of a tube, to establish the social hierarchy in weanling mice. Because the only requirement for mice to complete the tube test is the ability to move forward and backward inside the tube, the assay presents as a platform to investigate social ranking in young mice. Not surprisingly, our results indicated that young mice, just like adult mice, showed a stable social structure, and the hierarchical relationship can continuously last into their adult stages.
The establishment of the social hierarchy of weanling mice gave us an opportunity to investigate the relationship between ranking status and behavioral phenotypes in young animals. Unfortunately, our experiments cannot detect any relationship between social hierarchy and anxiety- or depression-like behaviors. However, we unexpectedly observed that dominant mice performed significantly better than subordinate mice in their memory abilities based on the novel object assay and the Y-maze assay. The greater long-term potentiation by electrophysiology and higher expression of memory-related genes by qPCR in the hippocampus of dominant mice also supported this association. To strengthen our finding, we further showed that memory-improving drugs can not only promote mouse memory, but also enhance their dominant status. Therefore, by taking behavioral, electrophysiological, molecular and pharmacological approaches, our studies suggested that mouse social dominance is highly correlated with their memory abilities.
Our findings in mice led us to ask whether the similar phenomenon could also be observed in young human children. This question motivated us to look for the help from the Department of Early Childhood Education, and eventually promoted this cross-discipline and cross-species research. We first applied a competitive bunny game with a comparable experimental design to the mouse tube test and successfully identified the social status of preschool children in a class. Next, we evaluated their memory abilities based on the “Picture memory subtest” and “Zoo subtest,” from the Wechsler Preschool and Primary Scale of Intelligence. Excitedly, we found that dominant young children, just like dominant young mice, possess better memory abilities. In addition to the behavioral assays, we also conducted inventory studies for dominant behaviors, resource control and memory ability of children, and the results further strengthen our conclusion that children dominance is positively correlated with their memory.
So, is there any functional consequence of better memory in the formation of social hierarchy? The survey on the resource control allowed us to not only evaluate resource control ability but also strategy usage. Interestingly, the correlation study showed that memory was only correlated with the resource control based on the prosocial strategies but not coercive strategies, suggesting that memory may play a role in the acquisition of prosocial strategies, which then improve the resource control ability. Last, to explore how social status affects children’s processing of social dominance cues, we designed an event-related potential (ERP) study and found a smaller FN400 amplitude in dominant children than subordinate children in response to dominant faces. Since FN400 is evoked by the processing of unexpected fluency compared with previous social knowledge, a smaller FN400 amplitude implies that children with a higher social rank probably have superior implicit memory in recognizing social dominance cues. Together, our data suggested that better memory could potentially contribute to children’s acquisition of higher social positions directly through processing dominant facial cues and indirectly through prosocial strategy learning.
In summary, our study revealed a remarkable similarity between humans and mice in the association between memory and social hierarchy. While the mouse model indicated the possible molecular and neural mechanisms underlying the association, the study in human children revealed the functional importance of memory in acquiring and maintaining dominance status. This research therefore presents a successful collaboration between life science and childhood education. To myself, as a biologist, it was a very unique experience to work with children and see our mouse data leading to a discovery in children behaviors. Particularly, the findings of this study provided an important reminder to our educators. To assist young children in achieving better social adaptation, we should take the level and limitations of children's cognitive development into account rather than just provide moral education or behavioral requirements. Most importantly, more attention should be paid to subordinates to ensure that every child has the opportunity to be listened to and the access to resources in the classroom.