When all those things travel together, it's not clear that polygamy should be viewed as the primary cause.
It’s not clear to me how the first three would lead to social instability though. There have been plenty of religiously authoritarian cultures that were quite stable. Similarly gender inequality and reduced education for women have been common historically and it’s hard to see how they would lead to social instability and violence. After all it’s not very common for women to rise up violently against gender based oppression, that just doesn't seem to be the dynamic.
I can certainly see how low productivity and standards of living could lead to violence. Desperate people can reach for desperate means. Nevertheless there are many very poor countries round the world and only some of them are significantly polygamous. The fact that the most unstable 20 are all to some degree polygamous seems to me to be a pretty strong correlation. If polygamy can be a driver of social instability though, it’s fairly straightforward to draw a logical causal link from that instability to all sorts of other social and economic ills.
As for conquest and Imperialism, Islam has strong polygamous tendencies and has been very aggressively expansionist in it's history. It even has formal rules spelled out in the Koran for acquiring women through conquest (their non-Muslim marriages are automatically rendered invalid on capture).
Funnily enough Muslim countries rank high on instability metrics, and we can see from recent history how stable and developed Muslim countries under autocratic rule collapse into chaos once that autocracy fails.
If even just the top 10% of men only have one extra wife, or 'girl friend' in a flat and secretarial role, as is common for middle eastern businessmen and politicians, that means the bottom 10% of men cannot ever marry. People will go to great lengths to get out of that bottom 10%.
I wonder to what extent the recent shift in sexual access for young men is a factor in political radicalisation in America. There seem to be a lot of angry young white kids around these days.
