Stellaris Ethos and Government
An example
For example - a Collectivist-Militarist-Materialistic empire would have access to six different types of government: MD, MJ, DH, SD, DE, and PO. The entire Democracy branch (MR, TR, DD, MD, and ID) is forbidden by their Collectivist trait, lack of Spirituality forbids DM+TO+TR, and lack of Pacifism forbids EM+PB+MD