The Editors would like to credit the following photographers and researchers for their photographs on the cover of this special issue:

• (Photographer credit; Researcher credit)

• Adam and David Tarnoki^©; Adam and David Tarnoki

• Andre Tan, Murdoch Childrens Research Institute, Heidi Wagner, Karin Allen, Vicki Kouts and Zaina Nehme^©; Jeffrey Craig and Richard Saffery

• Carlos Valiante^©; Kathryn Lemery-Chalfant

• Coral Pink^©; William Coventry

• Ellie Grant-Taylor^©; Nick Martin

• Frederic Volkringer^©; Juan R. Ordoñana

• Gillian Wilson^©; Svetlana Glinianaia

• Ilona Davies^©; Corinna Bennett

• Lisabeth F. DiLalla^©; Lisabeth F. DiLalla

• Nancy Segal^©; Nancy Segal

• Natascha Stroo^©; Natascha Stroo

• Sabrina Alriti, Franca D'Angelo and Belocchi^©; Sonia Bresciani

• Scott Bramwell^©; Gary E. Swan

• Sevgi Oncel^©; Fazil Aliev

• Shouko Taguchi and Chieko Temba^©; Syuichi Ooki

• Takayuki Fukudu^©; Kazuo Hayakawa

• Usa Duangvao^©; Somsong Nanakorn

The Editors would also like to gratefully acknowledge all the twins who appear in the images on the cover of this special issue.