The purpose of the study is to evaluate a 2-3 day treatment probe targeted to improving social gaze behavior in children with fragile X syndrome (FXS). The investigators will use the principles of Applied Behavior Analysis (ABA) to shape appropriate social skills. Importantly, the investigators propose to examine the effects of this treatment probe on brain and behavior.

Inclusion Criteria:

1. Boys diagnosed with fragile X syndrome
2. Boys with intellectual disability who do not have FXS or other known genetic disorder associated with intellectual disability
3. Age 8 to 18 years inclusive
4. Ability to travel to Stanford
5. Vineland Adaptive Behavior Scales standard score between 50 and 75 points

Exclusion Criteria:

1. Sensory impairments, or any other serious medical or neurological condition that affects growth or development (e.g., seizure disorder, diabetes, congenital heart disease).
2. Materials in body that would preclude an MRI scan (e.g., dental braces).