Zygosity testing should be encouraged for all same‐sex twins. (November 2015) - Craig, J.M.; Segal, N.L.; Umstad, M.P.; Cutler, T.L.; Keogh, L.A.; Hopper, J.L.; Rankin, M.; Denton, J.; Derom, Catherine A.; Sumathipala, A.; Harris, J.R.; the International Society for Twin Studies (ISTS) and the International Council of Multiple Birth Organisations (ICOMBO)