What is so hard to understand about this? Although its not good for society as a whole, it makes perfect sense on an individual level.
In the past, and even today in poor societies (which China still is if you look at PER CAPITA GDP), sons are assets, daughters are liabilities. A son protects the family, a daughter needs to be protected.
Even if we ignore that stuff, think about if your son or daughter goes out and has sex. If it was a son, most fathers will be proud that their son scored. If it was a daughter, even the most liberal minded fathers will not like the thought of it. |