Sorry, no April Fool’s joke this year.
But remember last week when I talked about making the 7th-son-of-a-7th-son fantasy trope make sense as a system? Well, it turns out I didn’t go nearly far enough. Flesh-Pocket, Melissa Walshe, and Peter Queckenstedt, and I fleshed out the mechanism for birth-order-dependent thaumaturism that generates all sorts of new and interesting problems.
Let’s say there’s an epigenetic factor (some kind of methylation?) that up-regulates magical ability with each successive pregnancy. The children of those pregnancies get more and more magical, and (if they’re women) they pass that magic along to their children in addition to their own methylation for THEIR successive pregnancies. After a certain number of iterations, the seventh child of a seventh daughter would pass some critical threshold and present as a sorcerer or sorceress. Boys might express magic, but they can’t pass it on.
There’s got to be a catch, though. The only reason a system like this would evolve is if the selective advantage of magic balances out some other disadvantage. Otherwise, everyone would be maximally magical. Perhaps magic comes at such a high metabolic cost that using it basically cripples you. So you have to have all of your six older siblings taking care of you to have any hope of survival. Of course you can fudge things by using medical magic to increase the chance that the child survives, but you run into diminishing returns there, too. 7th child of a 7th mother is where diminishing health and increasing magical ability balance out.
This flips the assumptions we make about medieval family-size, gender roles, and class. All women aristocrats will be expected to have at least seven children, but for commoners, having large families will be treasonous. Can’t have the peasants producing their own hedge-witches, after all.
Of course there’s no way to enforce small family size on the peasantry by law alone. Enforcing those laws means pissing people off enough that they’ll want to hurry up and produce a hedge-witch to overthrow you. The only way for feudal monarchs to make sure their peseants don’t over-reproduce is to make them work for money instead of farm for subsistence, using magic rather than mechanization to impose an industrial revolution and a demographic transition. So 7th Mother society will look a lot like our own, with social pressure for women to work and not have many children. Rebels will be the ones who hide their pregnancies and give birth in the basement.
Imagine a wheelchair-bound sorceress in her tower, waited on by jealous older siblings. A trivially magical 6ths son of a 5th daughter with no prospects sent out to seek his fortunes by invading neighboring kingdoms (who also have their own sorcerers). A common-born Chosen One produced to lead a rebel army and right wrongs that happened two generations ago, locked away from sight since infancy and trained as a child soldier. Welcome to the world of the 7th Mother.