Animal models have provided evidence that the gut microbiome may influence neurodevelopment and behaviors associated with anxiety disorders. Few studies, however, have examined this link in humans. There is correlational evidence for associations between the gut microbiome and anxiety, depression, and PTSD in adults, as well as some evidence of associations with childhood autism and temperament. Recent findings reveal that the gut microbiome significantly predicts fear behavior in 1-year-old children. The current study will be the first to examine the influence of early toxic stress, including the distal effects of living in poverty as well as the proximal factors of negative parenting and household chaos, on the development of gut microbiome diversity and maturity across 15, 24, 26, and 54 months. In addition, we will examine the bidirectional relationship between this development and that of behavioral inhibition at these ages and the HPA axis as mediators of these links. Changes in amygdala, hippocampus, and medial prefrontal cortex, using high resolution magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) and resting state fMRI (rfcMRI), will be tested as a mediator of the relationship between the gut microbiome and behavioral inhibition and anxiety measured at 4.5 years. Our long-term goal is to determine how colonization of the gut microbiome impacts human brain development and later risk for psychiatric illness research such that modulation of the gut microbiota could normalize neurodevelopmental trajectories early in the disease process, ultimately preventing the onset of psychiatric illness or reducing its severity. There is existing evidence of a sensitive period during which adverse experiences may alter the gut microbiota, and this has been posited to be between 0 and 4, so this study will provide important information that could inform intervention and prevention efforts early in life. This is an essential first-step in developing novel interventions to promote a healthy microbiome and reduce risk for psychiatric illness.