Some 21 million girls in India are "unwanted" and receive fewer resources because their parents wanted a son, the government said, as analysts called for action to boost women's earnings.
India has 21 million 'unwanted' girls due to preference for sons (timesofmalta.com)
If you write briefly about "actually what you wants to discover (aim or objective)" will make more easy to think.
However, you have two very specific question :
1. Sex preference
2.Reasons for reduction of research on this.
For 1, you divide the 195 contries (according to UN) search for regional Demography: you yourselfers revealed your answer. Answer would be empirically justified.
For 2, the answer of 1 will discovered little to understand the trends of male, female ratio (considered medical theory of foetus sex). Next to that, sex determination is prohibited in most of countries and reported case licences of doctor will band. Lastly, you can search other sectors like; medical research.
In African countries majority of people still prefer to have a son whilst the minority group especially the privileged ones no longer cares about the sex of the child
Thank you for the comment; apologies that I have not responded sooner!
I totally agree that this is the case even though we would like to think things have "improved".
Why is there less recent research? I think this is considered an "old" topic but certainly I encourage you to pursue this topic. Rafia Rahman has mentioned some very good suggestions for developing this topic! I think it would be very insightful to use your comment about it being less studied recently.