People around the globe often depend on informal financial arrangements, borrowing and lending money through social networks. Understanding this sheds light on local economies and helps fight poverty.Now, a study co-authored by an MIT economist illuminates a striking case of informal finance: In East Africa, money moves in very different patterns depending on whether local societies are structured around family units or age-based groups.That is, while much of the world uses the extended family as a basic social unit, hundreds of millions of people live in societies with stronger age-based…