||In the present article, we introduce the quadratic vagal activity–prosociality hypothesis, a theoretical framework
for understanding the vagus nerve’s involvement in prosociality. We argue that vagus nerve activity
supports prosocial behavior by regulating physiological systems that enable emotional expression, empathy
for others’ mental and emotional states, the regulation of one’s own distress, and the experience of positive
emotions. However, we contend that extremely high levels of vagal activity can be detrimental to prosociality.
We present 3 studies providing support for our model, finding consistent evidence of a quadratic relationship
between respiratory sinus arrhythmia—the degree to which the vagus nerve modulates the heart rate—and
prosociality. Individual differences in vagal activity were quadratically related to prosocial traits (Study 1),
prosocial emotions (Study 2), and outside ratings of prosociality by complete strangers (Study 3). Thus, too
much or too little vagal activity appears to be detrimental to prosociality. The present article provides the 1st
theoretical and empirical account of the nonlinear relationship between vagal activity and prosociality.