As the global population is about to cross 7 billion, demographers are warning that skewed gender ratios could be a major concern.

The biological standard for the sex ratio at birth of 104 to 106 males to every 100 females is what is prescribed by nature. But abnormal factors have resulted in deviations from these figures.

In India and Vietnam, the figure is around 112 boys for every 100 girls. In China, it is almost 120 to 100. The trend is spreading to regions like the South Caucasus, where Azerbaijan, Georgia and Armenia all post birth ratios of more than 115 to 100, and further west to Serbia and Bosnia.

Global awareness of the problem was raised in 1990 with an article by the Nobel Prize-winning Indian economist Amartya Sen that carried the now famous title: More Than 100 Million Women Are Missing. He calculated that there were around 100 million less women across Asia due to sex-selective abortion, infanticide or inadequate nutrition during infancy.

As many as half a million female fetuses are estimated to be aborted each year in India, according to a study by British medical journal The Lancet.

Sex-selective abortion is illegal in both China and India, but officials say the law is incredibly difficult to enforce.

The United Nations Children Fund said Tuesday that an estimated 5 million Bangladeshi women are missing due to social, cultural and economic conditions that cause an uneven male-to-female ratio.

As an outcome of this skewed sex ratio, some forecast an increase in polyandry and sex tourism while others predict cataclysmic scenarios with the rise of male-surplus societies where sexual predation, violence and conflict are the norm.