Julia Fischer

and female one, as Eric Vilain from the University of California at Los Angeles, USA, and colleagues have reviewed recently (Nat. Rev. Endocrinol. (2014) 10, 603–615). While these variations of human sex between male and female are not necessarily linked to the psychological issue of gender identity, both display a spectrum of many different shades that is much more complex than the traditional binary division where everybody has to be either male or female and accord to the stereotypical views of characteristic body appearance and behaviour. Science still has a lot to learn about the origins of both of these phenomena, which may share some causes such as the exposure to hormones during pregnancy. The highly sensitive nature of the matter and the ethical limits on experimentation and indeed interventions make it diffi cult to get to the bottom of the biological causes involved. Animal studies as well as exploration of the rich diversity of sexual reproduction in nature may help in this endeavour. Similarly, society at large is still struggling to come to terms with the new reality that sex and gender are not a black-and-white binary like the pictograms on public conveniences.