Tyler:

You are wrong about how Sen arrived at the 100 million missing women number--not by comparing the female to male ration in advanced countries with that of countries in South and South-East Asian countries. Here is what he says in the original article, "Missing women: social inequality outweighs women's survival advantage in Asia and north Africa, British Medical Journal, March 7, 1992 v304 n6827 p587(2):

'Using European or American ratios may not, however, be appropriate. Because of lower female mortality in Europe
and America the female:male ratio rises gradually with age. A lower ratio would therefore be expected in Asia and
north Africa partly because of a lower life expectancy and higher fertility rate. There are several ways of adjusting for
this. One is to adopt the female:male ratios of sub-Saharan Africa, where there is little female disadvantage in terms of relative mortality but where life expectancy is no higher and fertility rates no lower than those in Asia and north Africa. Using the sub-Saharan ratio
of 1.022 yields an estimate of 44 million missing women in China, 37 million in India, and a total of more than 100
million worldwide"

You should add a "correction". This is also a teachable moment. It suggests that one should not use metrics developed for rich countries to make judgment about poor countries. This issue shows up in the measurement of malnutrition.

To what extend might this missing women problem be a question of limited resources? Suppose boys have some economic advantage over girls in some region (for work in agriculture, for instance) and that same region is so poor that parents do often lack the resources to provide for all their children at a satisfactory level. Would it then not be rational, though gruesome, for parents to prioritize in nurturing boys?

I am asking because if this is at all a relevant factor, then trying to remedy the problem without first increasing the amount of resources available might make those poor societies actually worse off.

I think it is wrong to neglect social factors and reduce this problematic to diseases. Of course baby girls are being killed especially in China and India, there is even a term for it: Gendercide!
Video: http://www.causes.com/causes/789379-it-s-a-girl-documentary-end-genderci...
Statistics: http://www.nationmaster.com/graph/peo_sex_rat_at_bir-people-sex-ratio-at...