Idaho Code § 49-665: Riding on motorcycles.

A person operating a motorcycle shall ride only upon the permanent and regular seat attached to it, and the operator shall not carry any other person nor shall any other person ride on a motorcycle unless the motorcycle is designed to carry more than one (1) person, in which event a passenger may ride upon the permanent and regular seat if designed for two (2) persons, or upon another seat firmly attached to the rear or side of the vehicle.