Early development, temperament, and functional impairment in autism and fragile X syndrome.

Bailey et al. (2000) compared three groups of young boys: autism only, fragile X only, and boys with both. They looked at early milestones, daily living skills, and social-communication scores. The goal was to see which group had the most uneven or severe delays.

Boys with autism alone showed the most scattered skills—high in one area, low in another. Boys with fragile X alone had steadier but lower scores. Boys with both conditions scored worst in every area.

Olsson et al. (2001) followed the same boys one year later. They found that autistic behavior, not the fragile X protein level, predicted slower growth. The 2000 picture held up: autism features drive the bigger slowdown.

Waldron et al. (2023) repeated the autism-vs-fragile X contrast 23 years later with eye-tracking. Preschoolers with non-syndromic autism looked at faces even less than kids with fragile X, confirming the 2000 social gap.

Jaffe et al. (2002) tracked the same groups for three more years. IQ and daily-living scores dropped over time, with fragile X boys falling fastest. The 2000 cross-sectional snapshot turned into a downhill slope.

When you see a child with fragile X who also screens positive for autism, plan for the most variable and severe profile. Use short, face-to-face teaching trials and track adaptive skills every quarter. Share the data with families early; they will need the strongest support plan you can write.