Inclusion Criteria:

• * Diagnosis of IDD (IQ \< 70; social/adaptive dysfunction, onset \< age 22) as per DSM-5
• * Clinical diagnosis of Tardive Dyskinesia (TD) per DSM-5 for at least 3 months before study inclusion (presence of movement disorder for at least 3 months, in absence of previous formal diagnosis of TD).
• * Eligible to receive valbenazine according to current product labeling.
• * Stable doses of all psychotropic medications for minimum of three months before study inclusion.
• * Willing to remain on stable doses of all psychotropics for 24 weeks of study. If female of childbearing age, practicing acceptable form for birth control throughout study duration.
• * Subject able to comply with scheduled visits and assessments.
• * Consent of subject, or legally authorized representative to study protocol.

Exclusion Criteria:

• * Previous treatment with a VMAT2 inhibitor (tetrabenazine, valbenazine, or deutetrabenazine).
• * Treatment with any investigational drug in the 30 days prior to study entry.
• * Currently taking a strong CYP3A4 inducer such as carbamazepine, phenobarbital, diphenylhydantoin, or primidone.
• * Any unstable medical condition in the 60 days prior to study entry.
• * Pregnant or breast-feeding.
• * Inability to take study medication.
• * History of neuroleptic malignant syndrome.
• * History of long QTc on electrocardiogram, bundle branch block (BBB), atrioventricular block, serious cardiac arrhythmia, or heart failure.
• * QTc on EKG \> 450 msec (Fredericia formula) on EKG within 3 months prior to study entry.
• * History of substance abuse or dependence in the 3 months prior to study entry.
• * Significant risk of suicide or dangerous aggression to others at time of or 3 months prior to study entry.