why are the Shia more schismatic than Sunnis? or is that a false impression?

my not very informed impression is that whenever an unusual Muslim religious group with a distinct name shows up, it turns out that it is a branch of a branch… of the Shia faith. So it is as if the Sunnis are nicely dogmatically united throughout history while the Shia keep on splintering into these religious movements that sometimes manage to differentiate themselves to the extent that others deem them apostate.

Is there any validity to this view? Or are/were there actually lots of Sunni derived sectarian movements that I am being grossly ignorant of?

If the answer is yes, what is it about the Sunnis that make them more resistant to doctrine drift? Or, conversely, why is it that the Shia seem or seemed in the past so prone to it?

The Shia didn’t achieve much political success, except for the Fatimid Caliphate. I’d imagine that the Sunnis were more successful in killing off heretics. The Sunnis have 4 schools of thought (Madh’hab) but they are not really sects per se. It is true that some non- or semi-Muslim religions are though of as Shiite splinters or inspired by Shia, like the Alevis and Druze.

The big Sunni sectarian group you may have heard of are the Wahhabis. You may have heard of their actions in the last couple years…

It strikes me that, because there is not really a Sunni hierarchy, room exists for all kinds of doctrinal splits without causing anything like a schism. What in a more structured religion could lead to schisms in a more loosely structured group leads to schools. The Wahhabi are clearly a distinct group but they’re within general Sunni norms and there’s no one in particular who could kick them out. Also, it seems to me that in hierarchical religions, schisms are not infrequently driven by the politics associated with the hierarchy. Lack of hierarchy is one less reason for schism.

Further, if there is a within-group theological disagreement, the only two ways I can think of to know that a schism has occurred are (1) if the group declares itself separate–not likely to happen, as most schismatics know that they are keeping the pure faith, and (2) if something like a consensus of everyone else decides the group is out. In a hierarchical group, you are more likely to see a formal method for declaring a group out.

Shiism also seems to place great emphasis on knowing who was an Imam. That lends itself to the kind of semi-political dispute that produces schisms. Thus the what, niners and twelvers–I always get those various folks confused.

This is just an impression based on casual reading.

Come to S Asia, I’ll show you 2 Shia groups and about a dozen Sunni groups. Go to SE Asia or N Africa and they will say “Shia? Whats that”?

Off the top of my head, during my lunch break, with all my references on Shia’ history at home this is more or less it. Shia’ism was founded on the principle that God would not allow mankind to flounder with only a Book (the Koran) after Muhammad’s (pbuh) death. Rather, there would be a divinely appointed Guide, or Imam, coming from the family of the Prophet, who, whilst not able to “found” a new religion, would interpret God’s final message in a way that is appropriate for the particular time. Ali, Muhammad's cousin and son-in-law, was the first Imam, with tradition suggesting that this happened at a place called Gadhir-e-Kuhm, where the Prophet received his final revalation from God and declaring "to whoever I am the ultimate authority, Ali will be his ultimate authority". This basically established Ali as the first Imam to those who accepted this (and Shia’, IIRC, is derived from the Arabic phrase that translates to “Party of Ali”).

Once political posturing happened, internal conflicts etc, different groups of people believed that different people were the Imam, sometimes out of genuine belief, sometimes to forward political agendas. This gave rise to the Twelvers, the Druze, the Nizaris, and various other sects, most of whom believed at some point that the Imam went into occultation/hiding and would return. The Nizari Ismailis do still believe in a living Imam, but there’s not many of them around.