["Journal of Personality, EarlyView. ", "\nABSTRACT\n\nObjective\nPrevious studies showed that cardiac vagal regulation is quadratically associated with prosociality. Preliminary evidence also indicated that agreeableness, which is considered to reflect the propensity of prosociality, was quadratically associated with cardiac vagal regulation. However, more evidence is needed to demonstrate the quadratic association between cardiac vagal regulation and agreeableness. The present study investigated this issue with relatively large samples of Chinese middle school and college students.\n\n\nMethod\nAgreeableness Subscale from the Big Five Inventory‐2 was administered to middle school (N = 427, Mage = 13.18, SD = 0.58; 194 females) and college students (N = 401, Mage = 18.99, SD = 0.98, 266 females) to assess their agreeableness. The electrocardiogram data were collected to compute participants' resting respiratory sinus arrhythmia (RSA) as an indicator of cardiac vagal regulation.\n\n\nResults\nLow and high resting RSA were associated with low agreeableness; moderate resting RSA was associated with high agreeableness in both samples of middle school and college students.\n\n\nConclusions\nThese findings demonstrated a robust quadratic association between resting RSA and agreeableness across two Chinese samples, which deepens our understanding of the biological foundations of agreeableness and provides trait‐level evidence for the quadratic resting RSA‐prosociality relationship.\n\n"]