The broader autism phenotype in simplex and multiplex families.

Fahmie et al. (2013) asked two groups of parents to fill out the Broad Autism Phenotype Questionnaire. One group came from 'simplex' families—only one child has ASD. The other group came from 'multiplex' families—two or more children have ASD.

They wanted to see if the kind of family predicts how many autism-like traits show up in parents and siblings.

Parents and siblings in simplex families scored lower on every BAPQ subscale. They were less aloof, less rigid, and had better pragmatic language.

The gap was medium to large, meaning the two family types look clearly different when you screen for BAP traits.

Perez et al. (2015) extended the same idea to ADHD. They also found multiplex families pile up more cross-disorder traits, showing the SPX-MPX split is useful beyond autism.

Muller et al. (2022) flipped the lens: when parents themselves carry more BAP traits, they over-rate autism symptoms in their kids. This warns us to check parent BAP status before trusting caregiver reports.

Bora et al. (2017) used a data-driven method and still found a high-BAP parent group, mostly driven by communication problems. The three studies line up—communication quirks are the clearest marker.

When you intake a new client, ask if any siblings have ASD. A 'multiplex' answer flags that mom, dad, or even typical sibs may show subtle social-communication quirks. Expect them to need extra coaching to implement interventions faithfully. Also, double-check parent rating scales against teacher or clinician data—BAP traits can inflate scores.