This is a Phase 2 study designed to evaluate the efficacy and safety of ABT-126 in approximately 210 adults with schizophrenia. Subjects will be randomized to one of three treatment groups (ABT-126 Dose 1, ABT-126 Dose 2 or placebo) for a 12-week Treatment Period. The purpose of this research study is to find out whether ABT-126 compared to placebo can improve cognition and what side effects ABT-126 may cause. Cognition is the way a person thinks, and it includes abilities like paying attention, focusing, remembering things, and solving problems. Acronyms listed in the Outcomes and/or Eligibility sections for this study are defined below: • MCCB: Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) Consensus Cognitive Battery • UPSA-2: University of California at San Diego (UCSD) Performance-Based Skills Assessment-2 • CANTAB: Cambridge Neuropsychological Test Automated Battery • PANSS: Positive and Negative Syndrome Scale • NSA-16: Negative Symptom Assessment-16 • CGI-S: Clinical Global Impression - Severity