Growing Up in the Concrete Jungle: Urbanicity and the Neurodevelopment of Schizophrenia

Abstract

While schizophrenia is regarded as a neurodevelopmental disorder, the mechanisms through which exposure to the environment contributes to the onset of psychosis still lack a clear scientific explanation. The most well-established finding in epidemiology of the condition is a connection between growing up in the city and higher risks of the disease. However, urbanicity is commonly considered in terms of being a contextual rather than a developmental exposure that is associated with a biological response. Existing theories usually consider one component of schizophrenia in isolation without fully explaining its interaction. The main argument of this thesis is that urbanicity should be considered through developmental contexts in which stress experiences become biologically encoded. Based on findings derived from epidemiology, neurobiology, and stress sensitization literature, this thesis claims that the constant experience of stress in the urban environment triggers sensitization of stress response systems, specifically the HPA-axis and dopaminergic system. Such sensitization affects salience processing, leading to lower thresholds for experiencing a psychotic episode. Predictive coding theory is also employed to explain how these changes manifest. According to this theory, in the case of continuous stress, predictive systems responsible for adequately weighting and updating inputs from the environment can become faulty, leading to overestimation of certain stimuli. Building off this framework, chronic social stress in cities where the population is dense, fragmented, and unpredictable results in the formation of psychotic experiences in people who are predisposed to the disease. Unlike current models, this thesis attempts to conceptualize urbanicity as a collection of multiple developmental exposures that interact over time to shape a person's biological makeup and increase vulnerability to psychosis. This approach provides a comprehensive explanation of the effects of the environment on the neurodevelopment of psychosis.