I've heard that only the rich got wheat for their bread and porridge in premodern europe, while the poor made due with barley, oats, and rye. I always imaged there was an economic reason for this, such as that barley, oats, and rye had higher yields and that wheat was a prestige food with a lower yield but higher palatability. Is that true?
Were there other factors involved in wheat being the food of the rich?
Basically you are correct. Wheat has a smaller gross and net yield compared to barley or oats (and rye depending on the climate, soil, etc).
Wheat wasn’t more nutritious than its rival grains, but it was definitely preferred. Thus it had a higher price. In medieval England at least there were fields that were sewn with a more-or-less even amounts of wheat and barley, which is known as “mixtil”.
It’s fair to say that wheat was for the rich and oats and rye were for the poor, that’s a broad generalization. Barley is in a different category as it could be used to make beer or bread... or as animal fodder (wheat is the only crop we’re discussing that would almost never be used for animal fodder).
It could also be regional. When Samuel Johnson stated in his 1755 dictionary that oats in England were to feed horses, while in Scotland they fed the people, he was mocking the poverty of the Scots- and since he had some Scots working for him as lexicographers, he might have been indulging in a dig at them. But both oats and rye are more tolerant of poor soil and cold weather than wheat, so farmers in colder northern areas like Scotland would not necessarily have the choice of being able to grow wheat. A French peasant in the big grain lands around Chartres could eat bread, but a crofter in Sutherland might have no choice but to eat porridge and oat cakes.