How did more than 160 million women go missing from Asia? The simple answer is sex selection -- typically, an ultrasound scan followed by an abortion if the fetus turns out to be female -- but beyond that, the reasons for a gap half the size of the U.S. population are not widely understood. And when I started researching a book on the topic, I didn't understand them myself.  
 
I thought I would focus on how gender discrimination has persisted as countries develop. The reasons couples gave for wanting boys varies: Sons stayed in the family and took care of their parents in old age, or they performed ancestor and funeral rites important in some cultures. Or it was that daughters were a burden, made expensive by skyrocketing dowries.