[ramblenode]

Your example misses the point.

If there were evidence that driving on the right side or left side of the road slightly reduces car accidents and a country with previously no roads or cars began planning to automotize the country, then, all things considered, it would make sense to have people drive in the lane with a slightly reduced fatality rate.

If there are two choices where one presents a slight advantage but no additional cost then a rational actor will go with that choice.

[qwytw]

The event has to occur relatively frequently for that slight advantage to become statistically noticeable. Direct assaults on castles with hand-to-hand combat occurring in stairwells were extremely rare as far as we know.