Two factors influence stability.
In the 1950s, Saul Winstein proposed that the 2-norbornyl cation is a delocalized “non-classical” ion—sparking a decades-long debate with Herbert C. Brown. George Olah finally settled the argument using superacids and NMR spectroscopy to directly observe the cation at low temperature, earning the 1994 Nobel Prize and confirming Winstein’s picture of charge delocalization.