The evolution of <i>de facto</i> relationships in Brazil: from concubinage to unmarried unions

Abstract This work analyses the modernization of family law over the past few decades, using the social and legal transformations of marriage and informal relationships as its starting point. By tracing the historical evolution of marriage – from Roman law to contemporary models, with emphasis on some countries in Europe, the USA, and Latin America – it explores how legal frameworks have adapted to emerging family configurations. The research examines the legalization of same-sex marriage, the rise of unmarried unions, and the relaxation of formal requirements for the establishment of family units. In Brazil, the recognition of ‘stable unions’ (ie unmarried partnerships) as equivalent to marriage reflects this legal modernization. This is evidenced by the waiver of cohabitation as an essential criterion, as well as the distinctive legal structure adopted by the country. The varying approaches to the recognition of same-sex unions across countries also highlight the legal choices and cultural values influencing family law. Ultimately, the study argues that contemporary de facto unions share conceptual similarities with classical Roman marriage, particularly in their foundations of continuous consent and affectio maritalis, while differing markedly from the formalities of modern legal marriage.