How did "what" become "because"?

Question

Two of the most common words for "because" in Latin are quod and quia, both of which began as neuter forms of quī "who". (At some point quia got replaced with the feminine plural quae, though I don't know the reason for that.)

But this seems like a somewhat strange change. A shift from "because of which" to "because of what follows" makes sense, but to the best of my knowledge, reasons aren't generally given as bare accusatives in Latin: I can't say *eum feriī hoc to mean "I hit him because of this", I would need to add propter or something like that.

So: is it known how quod and quia turned into words for "because"? Were bare accusative reasons common in earlier Latin, for example, before dying out? Or is this a mystery lost to time?

Answer 1

This question is discussed in the following articles:

"Quod, Relative Pronoun and Conjunction", Robert B. Woolsey, The American Journal of Philology, Vol. 74, No. 1 (1953), pp. 52-69.

"Relative and Antecedent" E. Adelaide Hahn, Transactions and Proceedings of the American Philological Association, Vol. 95 (1964), pp. 111-141.

"When Quod Is "Which" and When Quod Is "Because"", Thomas Nelson Winter, The Classical Outlook, Vol. 72, No. 3 (SPRING 1995), pp. 79-84.

The general upshot of this is that the relative pronoun became causative either due to accusative specification or conditional apposition. You can read the articles above if you want to wade through the incredibly tedious details of this, which include involved discussions of Hittite.