Résumé : This paper investigates how social networks of group borrowers come into play in joint liability group lending. We use a large and original dataset containing 802 mapped social networks of borrowers from Pro Mujer Mexico. This is the first paper to look at external ties, i.e. social ties with individuals outside the borrowing group. Our main finding is that group lending with joint liability works when group borrowers use the informal risk insurance arrangement embedded in their external ties as guarantee for loan repayment. The extent to which this informal arrangement is used as guarantee is not decided by the borrower, but it is determined by the configuration of the group borrowers’ social networks, i.e. by their overlapping networks. These overlapping networks (or information channels) facilitate the diffusion of information into each other’s networks, and, thus, increases the credibility of the threat of losing one’s informal risk insurance arrangement in case of default. Our results show that the threat of losing the informal risk insurance arrangement embedded in one’s external ties matters for loan repayment even more than internal ties (i.e. ties between group members).