Portfolios of the Poor. The new book's four authors---Daryl Collins, Jonathan Morduch, Stuart Rutherford, and Orlanda Ruthven---took up an idea of David Hulme, to compile financial diaries of poor households. A researcher visits a poor household repeatedly, say, every fortnight for a year, and gathers detailed information on what its members earned, spent, borrowed, and saved since the last visit.

Much glory in the social sciences goes to those who study causality, who (seem to) show that A causes B. Yet the most enlightening work is often just plain descriptive, coming from a good, long stare at A or B.