Fullerton’s research focus is the identification of biological factors which increase risk to mental illness, with early postdoctoral work focused on personality, depression and anxiety, and more recent work focused on bipolar disorder and related psychotic and mood disorders.  Fullerton's foundational experience is in genomics and gene discovery via genome-wide association studies, genomic and transcriptomic sequencing. Recent work also includes understanding the role of changes in brain structure and function in risk of mental illness at diagnostic- or symptom-level, and analysis of population and life course data from administrative health records.