Abstract:

Third-party intervention refers to the proactive action of an uninvolved observer who, upon witnessing a behavioral event that violates, conforms to, or exceeds social normative expectations, imposes punishment, delivers rewards, or provides compensation to the actor involved. As a crucial mechanism for maintaining social order, third-party intervention has attracted extensive scholarly attention for its potential to foster prosocial behavior. Nevertheless, empirical findings remain inconsistent, suggesting that its effectiveness may depend on complex boundary conditions.

To systematically examine the strength of third-party intervention’s prosocial effect and its influencing factors, the present study employed a three-level meta-analytic approach, synthesizing 130 effect sizes from 40 empirical studies with a total sample of 10,289 participants. Results of the main effect analysis revealed that third-party intervention has a moderately strong positive impact on prosocial behavior. Moderator analyses further indicated that greater intensity and higher probability of intervention were associated with stronger prosocial effects. This pattern can be explained by several mechanisms: higher intensity and probability increase the anticipated costs of norm violations, enhance the potential benefits of engaging in prosocial actions, and transmit clearer normative signals, thereby fostering prosocial behavior. All forms of third-party intervention—punishment, reward, compensation, and their combinations—significantly promoted prosocial behavior, though the effect of rewards was relatively weaker. This asymmetry may be attributed to the principle of loss aversion, or the higher informational value of negative feedback relative to positive cues. Demographic moderators, including age and gender, were non-significant, suggesting that the prosocial impact of third-party intervention is stable across populations. Likewise, there was no difference between social versus monetary forms of intervention, indicating that both material incentives and social approval serve as fundamental motivation for prosocial behavior. Furthermore, whether the third-party agent was human or computer did not significantly alter prosocial effects. This may be because participants either lacked a strong psychological distinction regarding the agent’s identity, or because they focused more on the behavioral consequences themselves than on the intentions of the interveners. The presence or absence of intervention costs also did not significantly moderate the effect, which may reflect discrepancies between participants’ subjective perceptions and the coding schemes adopted by researchers. Future research should further investigate whether individuals’ prosocial behavior is differentially affected when they can clearly perceive variations in the level of intervention costs. Finally, methodological moderators, including measurement paradigms and control group setting, did not significantly influence outcomes, underscoring the robustness of the overall effect across experimental conditions.

Theoretical integration of these findings provides support for deterrence theory, focus theory of normative conduct, and indirect reciprocity theory. Specifically, deterrence theory explains the effect of punishment by emphasizing the increased costs associated with norm violations; focus theory highlights the capacity of intervention to draw attention to and strengthen normative awareness; and indirect reciprocity theory stresses the indirect reputational or cooperative benefits generated by intervention behaviors. Rather than being mutually exclusive, these perspectives are complementary, offering a multidimensional framework for understanding how third-party intervention fosters prosociality. Beyond theoretical insights, the findings also carry practical significance. Comparisons of control group conditions revealed that active third-party intervention consistently yielded stronger prosocial outcomes than either the absence of a third party or mere passive observation, underscoring its distinctive role in reinforcing social norms and sustaining cooperation.

In conclusion, this meta-analysis provides comprehensive evidence regarding the effectiveness and moderators of third-party intervention in promoting prosocial behavior. By clarifying the robustness of its effects, delineating key boundary conditions, and situating findings within complementary theoretical perspectives, the study advances our understanding of how external normative strategies can be leveraged to cultivate prosociality and collective well-being.

Key words: third-party intervention, prosocial behavior, three-level meta-analysis, moderating effect.