As India is poised to overtake China as the world's most populous country, we put questions from World Service listeners to the author of 8 Billion and Counting. Dr Jennifer Sciubba explains how the number of humans is growing in some countries, declining in others, how people are moving around the world and why that matters when it comes to money and work. She also discusses the issue of fertility and birth-rate, and it's close links to factors such as government support and childcare.

Presenter: Devina Gupta

(Image: A mother and child. Credit: Getty Images)