China and India aren't the only countries that show this trend. They are just the largest.
Why Some Parents Prefer Boys
In India and China, the birth of a son is cause for celebration. The family has gained a future asset: a child who can earn money for his parents and support them when they are old.
That's not the case for girls. "It's more expensive for a family to have girls than boys," says Charles Kenny, a senior fellow at the Center for Global Development. A boy has greater earning potential in these societies because there is a stigma against women working outside the home. And in India, when a daughter gets married, her family usually makes a generous donation of money and gifts to the groom's family.
So a daughter is seen as a drain on the family's resources. There's an Indian saying: Raising a daughter is like "watering someone else's garden." In other words, the benefits of raising a daughter will be reaped by the family the daughter marries into, not her own family.
In India, a girl is also more likely than a boy to die before she turns 5; that's the only country in the world where that's true. It's mostly to do with neglect. Biologically, girls have an advantage in the first few years of life. But in India, where the majority of people don't earn much more than $2.40 a day, scarce resources can mean a boy gets extra food and medical attention while his sister doesn't.