Strength and aggression are seen as masculine, sensitivity and insight are feminine – so when you can’t tell who’s male and who’s female, it can be confusing, disturbing and at times; frightening.

This is the story of America’s post-operative transmen, Thailand’s kathoeys, the Fa’afafine of Samoa, the sex-reassignment surgery capital of the world, Trinidad, Colorado, USA and a couple in a long-term relationship in which both partners are transgender.

Around the world, customs differ, but almost every society shares one thing the concept of gender. Many believe that there are only two: man and woman. But in India, transgender men who cut off their genitals live as women and form a third gender.

In Indonesia, hermaphrodite priests lead a society that recognizes five genders. And in rural Albania, women swap one gender for another to gain equality. Sometimes even the most conservative cultures must make room for those who challenge convention. But for many, embracing additional genders is still taboo.