Three celebrities go on a mission to discover the source of their defining trait, talent or characteristic. With the help of scientific testing, the latest psychology techniques, brain science and genetics, they attempt to answer a question that affects everyone – how do nature and nurture shape us?

In the first programme, John Barrowman embarks on a journey of self-discovery to find out the truth about his sexuality. John is a successful actor, musical performer, dancer, singer and TV presenter. For as long as he has been aware of his sexuality, John has been convinced that he was born gay and firmly believes that homosexuality is not a choice. But did nature really mould him or did nurture have a part to play?

John undertakes a series of tests including a DNA examination that is compared with his straight brother’s, and he takes to the streets of LA to try out a theory that the origin of homosexuality is linked to events in the womb that may affect the foetus.

Throughout the programme, as well as picking up clues from psychologists and scientists, John also meets some celebrities and extraordinary people who help shed light on the origins of their traits.

Thursday 24 July
9.00-10.00pm BBC ONE