Fig 1. Theoretical rationale behind the selection of BALB/cJ mice as study subjects.

Based on literature data indicating that BALB/cJ mice exhibit very low levels of social behaviour and on the hypothetical association between social behaviour and emotional contagion, we selected the upper and lower quartiles of BALB/cJ mice as our study population. The full line indicates the theoretical distribution of emotional contagion in a heterogeneous population of different mouse strains; the dashed line represents the theoretical distribution of BALB/cJ mice and the shaded rectangles represent our study population characterised by emotional contagion resistant (ECR; left rectangle) and prone (ECP; right rectangle) populations. Importantly whilst emotional contagion resistant theoretically represent an extreme population, emotional contagion prone subjects shall locate around the average of the general population.