Killed for being female?

Are 100 million women missing from the world? A listener asks More or Less to explore this powerful statement - "More girls were killed in the last 50 years, precisely because they were girls, than men killed in all the wars in the 20th century." The quote is from a book called 'Half the Sky' by Nicholas D. Kristof and Sheryl WuDunn. It has been used in articles, by UN agencies and on TV to highlight the fatal consequences of discrimination of women based on their sex. But is it true? More or Less looks at the evidence. How can we know if a woman is killed precisely because she is a woman? And how do we know how many men have been killed in war?