While scientists are concerned about overpopulation of Earth, Elon Musk says opposite scenario is a threat

Birth rates in developed nations have been plummeting for years, which has dragged down the overall rate. 

The average women had 2.5 children by 2020, half the rate of 50 years ago - rates are lower in the UK and US

Musk, and other concerned experts, say societies will end up with 'more grandparents than grandchildren' 

They say this will lead to 'myriad' of challenges, such as too few people to work, pay tax and look after elderly

However, while some nations set to see populations halve, others forecast to log a eight-fold increase by 2100