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Hello,
I have one question about serial mediation model. In my model, Y1 impacts Y2 via 2 mediators (M1 and M2) (cf. Photo). In every papers I read about serial mediation, they always analyze the effect of Y1 on M2 in the model. However, for my model, I don't have any theory base for the relation between Y1 and M2.
So that is my question : In serial mediation model, we have to analyze the effect of Y1 on M2 or this is not necessary ?

I hope that anyone can help me to be cleared about serial mediation model.
Thank in advance.

Usually, you also include the paths from Y1 to M2 and M1 to Y2 to show that you actually have a serial mediation and that the mediation is not only through M1 or M2. If you do not specify these additional paths you may find a serial mediation because you have specified an incomplete model. Thus, you want to specify the full model and show that it is actually the Y1->M1->M2->Y2 indirect effect that is significant. For this, you usually also specify a model where you change the sequence of M1 and M2 and show that this would not lead to a serial mediation.