If you multiply it out, the rules in Grain Into Gold seem to indicate that the fantasy farmer and his family (of mother, and two children) can produce enough food to feed themselves and seven other people. This means that just over one-third of the people were engaged in farming, but all the history books tell us that nine out of ten people were farmers. In some GIG circumstances, this number can go as low as 20% of the people were farmers. So what’s the deal? Well, it’s simple, though the math isn’t. The 1 in 5 or 1 in 3 numbers are dependent on a couple of issues. First, they assume that farmers only grow food. This obviously isn’t true as farmers also grow cotton, linen, sheep and goats for wool, hemp and various other textiles. They also grow spices, and though spices may not contribute to the “food” of the region, they were important crops that took labor and land. The farmers also grow all manner of specialty items such as tobacco, coffee, tea, indigo, etc ad nauseam. We didn’t even start on wine, beer and hooch.
Second, the Grain Into Gold production rates specifically do not take into account things like plagues of locust and droughts. Think of how much more food a farmer has to produce and store if every seven years a plague of locust wipes out pretty much all of his crops. This isn’t a silly notion, but part of the cycle of life. OK - so Americans may only have to worry about insect plagues every 17 years, but these farmers did not have insecticides to fight off these plagues. What about droughts? Depending on your definition of drought, they can occur as far apart as every 20 years or as close as every seven.
OK, so you need to plan out your world. You’re thinking - My world’s locust/cicadas are different from Earth’s, so I’m going to make them appear in huge numbers every ten years, because ten is an easy number to work with. And in this region of the world, where it’s usually pretty hot, I’m going to have a minor drought every ten years (easy math) and a major one every 20. So 1 in 10 years brings a reduction of 75% of food production (locusts), 1 in 20 bring s a 50% reduction (minor drought) and 1 in 10 brings a 80% reduction (we’re not planning to hit them for a minor drought and a major drought in the same year, so they’re really every 20 each). These events cut an average of 14% off the farmer’s production every year.
Lastly, and we’re not going to do the math on this, what about all those bandits out there? A nice calm country may not need to worry too much but an area that needs adventurers is likely going to see frequent bandit or orc attacks. Those damn bandits never just steal your stuff, they typically have to burn the crops and buildings. Not only are the crop lost, but if the farmer needs to spend time rebuilding, that’s time he likely isn’t growing stuff. What about taxes? Funny how I put bandits and tax collectors in the same paragraph, huh? Yes, the farmer is producing food, but the local lord is going to take some of it. This doesn’t change the fact that the local farmer is producing enough food for lots more people than live on his farm, but it shows how the redistribution happens. If the farmer is taxed at about 30% (20% civil and 10% religious), then his family of four is directly “sponsoring” one and a third governmental/religious person. Let’s look at our 3 out of 10 people are farmers. Well, 1 out of 10 is likely religious or supported by the religions. (Do they foster the poor?) 2 of 10 is government, likely one is a bureaucrat and one is a soldier. We just identified 60% of the population without including those who produce textiles or luxury goods, none of the miners, none of the craftsmen, and probably none of a lot of folks I’m not thinking of right now.
One final note - I never pretended that my fantasy worlds are like Earth at any age. Fantasy worlds need to be more than “The Middle Ages”. Whether it is magic or superior technology or the existence of elves, don’t let your players dictate something they learned in their history books. Earth history does not control your fantasy realms!