Actually, that's incorrect. There's a strong social prestige incentive for your one child to be a boy rather than a girl.
There's a strong economic incentive for you to have a girl: there's a tradition of reversed dowries in China, and girls are rare, so the benefits are high.
Perhaps that was the historical reason boys are high prestige, but modern China has huge opportunities for women (who "Hold up half the sky", as Mao put it).
China was a society of prima genitor, like Europe; boys are high prestige because they carry on your family name. All praise to the eldest son!
Infanticide for "economic" reasons is an evolutionary adaptation common in many mammals, including humans, including in rich societies. In the US, it usually gets classified as SIDS.
I'm yet to see a convincing argument for why it's unethical.
Isn't SIDS when a baby suffocates because they are sleeping on their stomach or in a room with poor ventilation? Or are you saying it isn't as sudden and accidental as we are led to believe?
The marriage ceremony is expensive - especially in the Middle East. And in countries like Egypt have low economic growth and huge inflation in marriage prices, so males have to work long and hard into their 30es to get married. And they can't just get a girlfriend and marry her in a inexpensive ceremony with only a civil unions - the religious leaders made sure its illegal (they have a interest in preserving the expensive traditions, you see).
Most men in the Middle East can hardly afford a single wife. Only really, really rich men have harems.
This is discussed in Jared Diamond's book "Collapse." (A good book, although winding.) A skewed sex-ratio was definitely one of the factors, but it would be foolish to say it was the only one. In fact, one of the theses of "Collapse" is that it is almost always a combination of multiple factors that leads to major changes in a society.
:)
(ratio of men vs women is one of the highest in the world due to single child policy)