High cardiac vagal control protects against future depressive symptoms under conditions of high social support.

High cardiac vagal control (as measured by respiratory sinus arrhythmia; RSA) is associated with lower depression. Recent theories argue that people's responsiveness to social resources is a key mechanism in this association. This argument implies two hypotheses: first, high RSA should be related to decreased depressive symptoms only when social resources (social support) are available; second, utilization of available social resources (social engagement) should serve as a mechanism for the positive effects of RSA. To test these hypotheses, we measured RSA in 131 adults. Participants reported their social support, social engagement, and depressive symptoms. Six months later, they again reported their depressive symptoms. Participants with higher RSA reported fewer depressive symptoms six months later, but only under conditions of high social support. The interaction between RSA and social support in predicting depressive symptoms was fully mediated by social engagement. These findings provide crucial support for the idea that cardiac vagal control contributes to decreased depressive symptoms via social processes. Implications for biological sensitivity to context and differential susceptibility theories as well as for the prevention and treatment of depression are discussed.