One way groups can be classified is the naturality of forming the group.
Naturally forming groups of people acting in pure interest in other members just might be able to force out unfavorable members. Such as friend groups.
On another end there is very artificial groups. I would consider astronauts these. Enough options and time to pick out the well- fitting ones.
But on the valley there is the majority of the groups, which are grouped around agenda, be it idea, hobby or profession. Coppers, locksporters, religious groups, Swedes and men named Tom. When the focus is not in the internal nor external selection of members, but gathering around a mutual thing or task, there will be unfit members.
Thus it’s not matter of ‘if’, but rather ‘when’ and ‘how do we react?’
TL;DR: Groups with common interest are susceptible to unfit people.
See, your theory fits within the framework of OP’s. OP is suggesting that, in friend and highly-competitive groups, the twat-ratio is much lower, say in the lower tenth percentile. In OP’s “valley” of interest and professional groups, the ratio may be closer to your 50:50.
So, we could form a Unified Twat Theory, where the ratio of twats-to-competence is inversely proportional to the severity of the selection criteria.
I leave the proof as an exercise for a grad student.