Stegoceras sported a 7,5 cm-thick skull. It was initially proposed that male Stegoceras (and individuals of other pachycephalosaurid species) would ram each other headlong, not unlike contemporary bighorn sheepor musk oxen. It was later suggested that they engaged in flank-butting rather than ramming, a widely evidenced theory. Foremost, the rounded shape of the skull roof would lessen the contacted surface area during head-butting, resulting in glancing blows.
Second, pachycephalosaurs would not have been able to align their head, neck, and body in a perfect horizontal line (which would be needed to transmit stress) -- it was more likely that they carried their neck in an "S"- or "U"-shaped curve (Stegoceras seemed to carry their spine in a less extreme curve, due to their thick neck muscles).
Lastly, the relatively large width of most pachycephalosaurs would have served to protect vital organs from harm during flank-butting. When a partial skeleton of Stegoceras was first discovered, it was thought to have gastralia, or belly ribs, not typically found in other ornithischian dinosaurs. They were subsequently found to be ossified tendons.