ZHENGZHOU, Nov. 14 - The number of males in China at marriage age is 18 million more than that of females due to a long period of high sex birth ratio since the 1980s, according to the country's family planning authorities.The sex ratio at birth in rural areas is 122.85:100, higher than the national average of 119.58:100, as compared with the normal sex ratio of 103 to 107:100, according to Zhang Weiqing, National Population and Family Planning Commission director.By 2020, males aged between 20 and 45 are forecast to be 30 million more than females in the country, he said at a rural population and family planning conference in the Henan Province capital.Rural families still have a preference for boys as agricultural production currently relies mainly on laborers, according to Zhang.China will continue to crack down on illegal prenatal sex selection and will try to help people discard traditional ideas of a preference for boys, the official said.