Men will dramatically outnumber women within decades because of 'cultural preferences' for boys and prenatal sex selection, a study has found.
The paper, published in the journal BMJ Global Health, blamed cultural preferences for boys for causing uneven ratios and projected a 'conservative' deficit of 4.7million female births by 2030.
Researchers also said prenatal sex selection has accounted for about half of the recent female deficit, creating skewed ratios at birth in countries across Southeast Europe to South and East Asia since the 1970s.
Researchers warned over one third of the world's population would be left with long-term sex imbalances as a result, with unknown social and economic impacts on affected countries.
Lead author Dr Fengqing Chao said: 'Fewer than expected females in a population could result in elevated levels of antisocial behaviour and violence, and may ultimately affect long-term stability and social sustainable development.'
A male-biased sex ratio could also result in marriage squeeze - where many men do not marry because there are not enough women.
Researchers said understanding the potential evolution of sex imbalances at birth was 'essential' for anticipating and planning for changing sex structures across the world.
The main challenge, the paper said, is to understand whether birth masculinity will stay indefinitely skewed in countries affected by sex-selective abortions and whether new countries may be affected in the future.
Dr Chao called for 'broader legal frameworks' to establish gender equality.
Sex selection is one of the key harmful practices defined by the United Nations and targeted under the Sustainable Development Goals.
The results were projected in a global sex ratio model using over three billion birth records from 1970 to 2020 from 204 countries.
The data also included records from countries, such as China and India, which already have an uneven birth sex ratio.
The results showed several sub-Saharan countries, Nigeria, and Pakistan, were expected to have a skewed sex ratio in the coming years - but projected the rate was likely to plateau and then decline in two decades.