The goal of this study is to find out if brain stimulation can help people stop skin-picking or nail-biting. The study wants to answer two main questions:

1. Does brain stimulation reduce the urge to pick skin or bite nails after those urges are triggered?
2. Does brain stimulation reduce how often people pick their skin or bite their nails?

Participants will:

• Talk about their skin-picking, nail-biting, and other mental health concerns
• Be placed in situations that make them want to pick or bite
• Rate how strong their urges are before and after brain stimulation Researchers will compare real brain stimulation to a placebo (a fake version that looks the same but has no effect) to see if the real stimulation works to reduce skin-picking and nail-biting urges and behaviors.

Inclusion Criteria:

• Ability to provide informed consent (as established by clinical interview), and voluntary, signed informed consent prior to the performance of any study specific
• Ability and willingness to perform study-relevant clinical assessments and tDCS
• Endorses mild to extreme intensity of urges to pick or bite their skin or nails

Exclusion Criteria:

• Any unstable medical, psychiatric, or neurological condition (including active or otherwise problematic suicidality) that may necessitate urgent treatment
• Daily use of psychotropic medications that substantially lower seizure threshold (e.g., clozapine)
• History of psychosis, mania, major neurological disorder, significant head trauma, or seizures/epilepsy
• Current suicidal intent
• Any major neurological disease or history of major head trauma, including concussion with extended loss of consciousness, or of psychosurgery
• Current or suspected pregnancy
• Endorsing possible contraindications for tDCS