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We introduce and investigate the philosophical concept of ‘speciesism’—the assignment of different moral worth based on species membership—as a psychological construct. In five studies, using both general population samples online and student samples, we show that speciesism is a measurable, stable construct with high interpersonal differences, that goes along with a cluster of other forms of prejudice, and is able to predict real-world decision-making and behavior. In Study 1 we present the development and empirical validation of a theoretically driven Speciesism Scale, which captures individual differences in speciesist attitudes. In Study 2, we show high test-retest reliability of the scale over a period of four weeks, suggesting that speciesism is stable over time. In Study 3, we present positive correlations between speciesism and prejudicial attitudes such as racism, sexism, homophobia, along with …