In the third and final edition, twin doctors Chris and Xand van Tulleken set out to explore how children develop a sense of their own identity and whether girls are really different to boys. The van Tullekens take a group of British under-sevens away from their families to an island in the middle of the River Thames, to take part in experiments to find out what it means to be a girl or a boy. Twin sisters Tyiana and Taiya shock themselves and the boys in a fairground competition and four-year-old American pageant queen Lyndsey has very strong ideas about how she likes to look.