The inhibitory
gamma-aminobutyric acid (
GABA) system is involved in the etiology of most
psychiatric disorders, including
schizophrenia,
autism spectrum disorder (ASD) and
major depressive disorder (MDD). It is therefore not surprising that
proton magnetic resonance spectroscopy ((1) H-MRS) is increasingly used to investigate in vivo brain
GABA levels. However, integration of the evidence for altered in vivo
GABA levels across
psychiatric disorders is lacking. We therefore systematically searched the clinical (1) H-MRS literature and performed a meta-analysis. A total of 40 studies (N = 1,591) in seven different
psychiatric disorders were included in the meta-analysis: MDD (N = 437),
schizophrenia (N = 517), ASD (N = 150),
bipolar disorder (N = 129),
panic disorder (N = 81),
posttraumatic stress disorder (
PTSD) (N = 104), and
attention deficit/hyperactivity disorder (
ADHD) (N = 173). Brain
GABA levels were lower in ASD (standardized mean difference [SMD] = -0.74, P = 0.001) and in depressed MDD patients (SMD = -0.52, P = 0.005), but not in remitted MDD patients (SMD = -0.24, P = 0.310) compared with controls. In
schizophrenia this finding did not reach statistical significance (SMD = -0.23, P = 0.089). No significant differences in
GABA levels were found in
bipolar disorder,
panic disorder,
PTSD, and
ADHD compared with controls. In conclusion, this meta-analysis provided evidence for lower brain
GABA levels in ASD and in depressed (but not remitted) MDD patients compared with healthy controls. Findings in
schizophrenia were more equivocal. Even though future (1) H-MRS studies could greatly benefit from a longitudinal design and consensus on the preferred analytical approach, it is apparent that (1) H-MRS studies have great potential in advancing our understanding of the role of the
GABA system in the pathogenesis of
psychiatric disorders. Hum Brain Mapp 37:3337-3352, 2016. © 2016 Wiley Periodicals, Inc.