This was an anecdote in some business book I'm currently reading (A World without Email, Cal Newport) and it's not crucial to the point the author is making, but it sounds fascinating nonetheless.
He quotes some historian from the 60s or so to argue the following:
Is this school of technological determinism still relevant, or is this waaaaay off, similar to Jared Diamond's geographical determinism?
Ah, the Stirrup Theory! It's one of those things that has been soundly disproven in academia-land, but still lingers in popular culture. It's also wrong in both its historical claims and its practical underpinnings, as we will see. (Indeed, effective lance-armed heavy cavalry were already around before the Western European knight; cue angry cataphract noises.) As always, more can be said if anyone would like to go at Lynn White's thesis; for the meantime, here are some previous posts on the matter:
In addition to these, some practical demonstrations, specifically Richard Alvarez's tests and this paper by Alan Williams, David Edge, and Tobias Capwell. The stirrup is certainly helpful, nobody's saying it isn't, but it's overhyped. To put it in gaming terms, stirrups are a veterancy upgrade to cavalry units, they don't unlock any new units. (Funnily enough, Attila Total War does exactly this in its Charlemagne campaign, heh heh heh...)