Impact of Social Media Influencer Marketing on Consumer Behaviour: A Review of Source Credibility Theory

Social media influencer marketing (SMIM) is now a dominant digital communication strategy shaping consumer decision-making, yet prior SMIM research remains theoretically fragmented and methodologically dispersed, limiting a coherent understanding of how influencer attributes drive behavioural outcomes. This structured literature review synthesises evidence from 17 peer-reviewed journal articles published between 2014 and 2024. Guided by Source Credibility Theory, the review analyses the roles of influencer attractiveness, expertise, and trustworthiness as antecedents of perceived credibility. Descriptive mapping, thematic synthesis, and contextual comparison were used to consolidate key themes, study designs, variables, mediators, and moderators. Across studies, attractiveness, expertise, and trustworthiness consistently strengthen perceived credibility. Higher perceived credibility is repeatedly associated with stronger consumer trust, engagement, and purchase intention. Influencer-brand fit and disclosure transparency emerge as critical determinants that condition campaign effectiveness and shape how credibility translates into consumer responses. The synthesis clarifies core credibility pathways in SMIM and proposes an integrative framework to reduce fragmentation and guide future theory development. Practically, the findings highlight the importance of aligning influencer selection with brand fit and ensuring authentic, transparent content. Key gaps include limited theory-building, narrow contextual coverage, and heavy reliance on cross-sectional self-reports, indicating the need for longitudinal and behavioural-data research to strengthen causal inference and external validity.