Like most sexual species, the sex ratio in humans is close to 1:1. In humans, the natural ratio between males and females at birth is slightly biased towards the male sex, being estimated to be about 1.05[2] or 1.06[3] or within a narrow range from 1.03 to 1.06[4] males/per female born. Sex imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants,[5][6] war casualties, sex-selective abortions, infanticides,[7] aging, femicide and problems with birth registration.[2]