The abortion of female fetuses and infanticide was aided by the spread of cheap and portable ultra-sound scanners in the 1980's. Illegal mobile scanning and backstreet hospitals can provide a sex scan for as little as $50, according to one report. "Prenatal sex selection was probably the primary cause, if not the sole cause, for the continuous rise of the sex ratio at birth," said population expert Prof. Chu Junhong.

In a 2002 survey conducted in a central China village, more than 300 of the 820 women had abortions and more than a third of them admitted they were trying to select their baby's sex.

According to a report by the International Planned Parenthood Federation, the vast majority of aborted fetuses, more than 70 percent, were female, citing the abortion of up to 750,000 female fetuses in China in 1999.

A report by Zhang Qing, population researcher of the Chinese Academy of Social Sciences, said the gender imbalance is "statistically related to the high death rate of female babies, with female death rate at age zero in the city or rural areas consistently higher than male baby death rate." Only seven of China's 29 provinces are within the world's average sex ratio. Zhang Qing's report cited eight "disaster provinces" from North to South China, where there were 26 to 38 percent more boys than girls.

In the last census in 2000, there were nearly 19 million boys more than girls in the 0-15 age group. "We have to act now or the problem will become very serious," said Peking University sociologist Prof. Xia Xueluan. He cited the need to strengthen social welfare system in the countryside to weaken the traditional preference for boys.