From a purely methodological perspective, I think it is interesting to note that this study does not account for whether the monozygotic twins share the same placenta.
75% percent of monozygotic twins share the same placenta (“monochorionic twins”). , whereas 25% have separate placentas (“dichorionic”). If the twins are monochorionic, there is a real risk of twin-to-twin transfusion syndrome, such that one child steals resources from the other in utero.
If you are studying an outcome that is affected by twin-to-twin transfusion (such as intelligence), including monochorionic twins in the sample will cause substantial bias towards lower correlation. Therefore, if I designed the study, I would have looked only at dichorionic twins. Obviously, this means you would have to throw out 75% of the sample, but I think they should have at least included some figures on how many of the twins were monochorionic, with perhaps a separate analysis using only dichorionic twins to see whether it affected the conclusions.
From a purely methodological perspective, I think it is interesting to note that this study does not account for whether the monozygotic twins share the same placenta.
75% percent of monozygotic twins share the same placenta (“monochorionic twins”). , whereas 25% have separate placentas (“dichorionic”). If the twins are monochorionic, there is a real risk of twin-to-twin transfusion syndrome, such that one child steals resources from the other in utero.
If you are studying an outcome that is affected by twin-to-twin transfusion (such as intelligence), including monochorionic twins in the sample will cause substantial bias towards lower correlation. Therefore, if I designed the study, I would have looked only at dichorionic twins. Obviously, this means you would have to throw out 75% of the sample, but I think they should have at least included some figures on how many of the twins were monochorionic, with perhaps a separate analysis using only dichorionic twins to see whether it affected the conclusions.