The chapter aims to analyze the rise of outsider deputies in the Chilean and Brazilian parliaments following the elections of maverick presidents, Jair Bolsonaro and Gabriel Boric, respectively. It investigates the characteristics and distribution of these outsiders, the factors influencing their election, and the implications for political systems in both countries. The authors introduced an innovative “outsiderism” index, employing Item Response Theory (IRT) to assess the extent to which elected parliamentarians are political outsiders. This novel index evaluates variables such as prior political experience, party affiliation, and involvement in political activism. The study analyzed data from the 2018 Brazilian and 2021 Chilean lower house elections, focusing on candidates who had not previously held office. The analysis revealed significant differences between the countries. In Brazil, a majority of outsider deputies were affiliated with right-wing parties. The Chilean parliament had a more balanced distribution, with a notable presence of independent outsiders. The outsiderism index showed higher average scores for Brazilian deputies, indicating a greater prevalence of complete outsiders. The findings suggest an association between the election of maverick presidents and the increased representation of outsider parliamentarians, particularly in contexts of declining trust in political institutions.