Why are males and females 50%-50%?

[JCMach1]

43. Except now in some places... like India and China

women are choosing to selectively abort female fetuses...

that can change the ratio game mightily...

Most people know China and northern India have unnaturally large numbers of boys. But few appreciate how bad the problem is, or that it is rising. In China the imbalance between the sexes was 108 boys to 100 girls for the generation born in the late 1980s; for the generation of the early 2000s, it was 124 to 100. In some Chinese provinces the ratio is an unprecedented 130 to 100. The destruction is worst in China but has spread far beyond. Other East Asian countries, including Taiwan and Singapore, former communist states in the western Balkans and the Caucasus, and even sections of America’s population (Chinese- and Japanese-Americans, for example): all these have distorted sex ratios. Gendercide exists on almost every continent. It affects rich and poor; educated and illiterate; Hindu, Muslim, Confucian and Christian alike... http://www.economist.com/node/15606229