The social construction of gender-as-binary plays an increasingly central role within gender equality research and activism. Despite its importance, however, there remain few empirical tools for assessing binarist beliefs, practices, or behaviours at the individual level. This thesis sought to address this gap by, first, proposing a new way to operationalize the gender binary, second, introducing the Implicit Relational Assessment Procedure (IRAP) as a potentially valid and reliable psychometric measure of automatic binarist beliefs. The current work had three broad aims: First, it aimed to conduct a comprehensive survey of self-reported and automatic binary beliefs in a sample of young Irish adults. Nine separate studies were conducted in the service of this (N = 602), which together provided clear evidence that gender is indeed structured in a binary, oppositional way (i.e., women are feminine but not masculine and men are masculine but not feminine). They also provided novel insights into the relational structure of gender roles, and the asymmetrical way in which we “gender” men relative to women. A second aim of this work was to examine the role of the binary in inequality. To this end, studies examined the relationship between IRAP effects and responses on three different measures of gender discrimination and prejudice: gendered hiring preferences (Chapter Three), androcentric bias (Chapter Four), and sexual harassment proclivity (Chapter Five). While studies in Chapter Three provided strong evidence that the binarisation of gender underpins discrimination in occupational contexts, effects in the remaining chapters were comparably weaker. Lastly, this thesis took the novel step of gathering a sufficiently large IRAP dataset for a set of pooled analyses. These analyses (Chapter Six) strengthened the conclusions drawn around the strength of the biases on the binary IRAP, provided novel insights into the magnitude and nature of gender differences on this measure, and shed light on some of its psychometric properties. Overall, these findings have a number of broad implications: First, they add to the growing empirical literature around binarist ideologies and their role in gender inequality. Second, they inform our understanding of how gender is structured, and elucidate the oppositional, relational, and asymmetrical way in which gender categories are framed. Third, they reveal the IRAP to be an adequately reliable and valid tool for quantifying gender binary biases. Fourth, and last, they attest to the automaticity of binary beliefs and thus the centrality of the binary within gender cognition more broadly.