Towards a unified theory of emotional contagion in rodents—A meta-analysis.

• Rats and mice show comparable levels of emotional contagion. • Rats and mice show an equivalent contagion response to familiar and unfamiliar conspecifics. • Prior experience with an emotional stimulus increases fear contagion response in rats but not in mice. • Social testing condition influences the level of contagion. Here we leverage 80 years of emotional contagion research in rodents and perform the first meta-analysis on this topic. Using 457 effect sizes, we show that, while both rats and mice are capable of emotional contagion, there are differences in how various factors modulate empathy in these species: 1) only mice show strain-specific differences in emotional contagion response; 2) although rats and mice have equivalent contagion response to familiar and unfamiliar individuals, our results show that familiarity length is negatively correlated with level of contagion in rats only; 3) prior experience with emotional stimuli almost doubles fear contagion response in rats while no changes are detected in pre-exposed mice; 4) both mice and rats tested alone show comparable reduced contagion compared to animals tested in a group; 5) emotional contagion is reduced in animals from both species missing one sensory modality compared to situations where all sensory modalities are recruited during emotional contagion. Lastly, we report similar patterns of brain activation during emotional contagion in rats and mice. [ABSTRACT FROM AUTHOR]