The other reason specific to a few Asian societies is because a combination of traditional preference for sons and the availability of sex-selective abortion skewed the sexratio at birth 20 years ago, leaving too few native-born women now.
The 2005 "mini-census" reported that the sexratio at birth, expressed as the number of boys per 100 girls, was 119--when the global average was somewhere between 103 to 106.