Research indicates multiple genes likely contribute to AS and autism spectrum disorders, but genetics alone don't tell the whole story. **Twin study evidence:** In identical twins (same genetic makeup), if one twin is diagnosed with autism, the second twin has only about a **60% likelihood** of having the diagnosis. This gap strongly suggests environmental factors play a role. What those environmental factors might be is still unknown — research is ongoing. The multifactorial model (genes + environment) is the current consensus, replacing earlier single-cause theories. This has implications for: understanding why AS rates appear to be increasing (better diagnosis vs. genuine increase vs. environmental triggers), and where research funding should be directed. **Source:** Lovett, *Solutions for Adults with Asperger Syndrome*, Ch 5 (p126) See also: [[History of AS Diagnosis]]