On my way to the World Happiness Summit in London, I receive a Whats-App video from my brother. My niece, five-year-old Ada, is moulding Play-Doh, singing under her breath. ‘What’s the song?’ asks her father from behind the camera. ‘In-es-cap-able de-ath,’ she chants, following up with a quicker, zanier, and more jaunty: ‘Inescapable death.’