There are divergent perspectives on how to conceptualize kindness in the existing literature, as well as a lack of conceptual clarity and theoretical integration. This entry paper centers on the conceptual framework of positive psychology, within which kindness is considered a psychological strength in a prosocial relational context. According to the Values in Action (VIA) model [1], kindness is within the virtue of humanity and is among the “interpersonal strengths that involve tending and befriending others” (cited in [2], p. 25). Within this framework, kindness can be studied as a trait, as a stable personal characteristic, or a state, manifested as a momentary reaction, or a behavior/skill, which can be acquired or cultivated in supportive environments. Also, kindness is not a singular construct but a multifaceted concept that includes three core components: kind emotions, kind cognitions, and kind behaviors [3].

By taking a developmental lens, the importance of recognizing and fostering kindness in developmentally sensitive periods, such as childhood and adolescence, is emphasized in this paper. Early studies in developmental psychology focused on prosocial behavior and altruism in children, laying the groundwork for understanding kindness as a distinct construct. Kindness emerges early in childhood, with prosocial behaviors observed in infants as young as 14 months of age [4,5]. The capacity for kindness increases with age, becoming more sophisticated and intentional throughout childhood and adolescence [6,7]. Studies have consistently shown that engaging in acts of kindness, such as prosocial behavior, is associated with healthy development, especially enhanced social connections and well-being in both children and adults [8,9,10]. Given the benefits of kindness, research has examined interventions of kindness in various settings, such as schools, clinics, and workplaces, offering evidence-based strategies for promoting kindness. In this entry paper, we focus on kindness in school contexts and kindness interventions for school-age populations. Also, though most kindness research has been conducted in North American contexts, some studies were conducted in diverse regions or cultural contexts. Findings that expand the understanding of kindness’s conceptualization and its implications in promoting kindness in children and adolescents are incorporated.