Befriended to polarise? The impact of friend identity on review polarisation-A quasi-experiment

Opinion polarisation in social media has recently become a significant issue. The existing literature mainly attributes polarisation to online friends' informational social influence, that is, users are more likely to interact with others with similar opinions, which leads to the echo chamber effect. However, the impact of social interaction on individual polarisation may also result from normative social influence, which varies with social settings on the platform. In this paper, we leverage a quasi-experiment to investigate the normative social influence of online friends on focal users' review polarity. We use fixed effects and difference-in-differences approaches, along with propensity score matching, to address the potential endogeneity in users' friend function adoption decisions. Our results indicate that adopting the friend function leads users to post less extreme ratings. We further separate the reviews into positive and negative, finding that the reduction in the review polarity for positive reviews is more prominent than for negative ones. Regarding user heterogeneity, our causal forest analysis uncovers that users with a higher engagement level on the platform are less affected by adopting the friend function than those with less engagement. Our study has clear implications for managers and platform designers, highlighting the importance of social function design in reducing social media induced polarisation.