Schizophrenia and other psychoses are disabling disorders with complex phenomenology and aetiology. Over the past decade, significant advances in understanding psychotic symptoms, delusions in particular, have been achieved. The key cognitive factor that has been suggested to contribute to the development of delusions in individuals with increased liability to psychosis is reasoning biases, including a tendency to jump to conclusions (JTC) and belief flexibility. However, to date, there is only very limited evidence on their role as a causal mechanism in delusion formation. In order to investigate whether reasoning biases (i.e. JTC bias, belief flexibility) are a causal mechanism in the development of delusions, I will be the first to draw on, and combine, three innovative approaches: 1) a real-life temporal association approach, to investigate moment-to-moment variation in, and temporal priority of, reasoning biases over delusions in daily life; 2) an interventionist-causal model approach, to examine whether experimentally induced changes in reasoning biases subsequently lead to changes in delusions in daily life; and 3) a psychometric psychosis liability approach, to investigate whether these effects are modified by psychosis liability. Building on my own, ongoing research, I will use the Experience Sampling Method (ESM) to investigate real-life temporal associations between reasoning biases and delusions in individuals with low, intermediate, and high liability to psychosis. Further, I will test whether, in individuals with high liability to psychosis, experimental exposure to metacognitive training (MCT) reduces reasoning biases and, in turn, delusions in daily life. The proposed research will deliver a step change in our understanding of reasoning biases as a causal mechanism in delusion formation, thereby, contributing to enhance effectiveness of psychological treatments under real-world conditions and, ultimately, improve psychosis outcome.