I am running models of GxE where I have age as a continuous moderator of a continuous phenotype. I have initially run the standard linear GxE model as outlined by Purcell (2002). However, I had problems getting this to fit. Out of interest, I ran the non-linear script, and this was successful. In addition to running the full model I have run nested models where the influence of the moderating terms are successively dropped. I am just wondering whether it is logically implausible to run a model where I retain quadratic paths (for the non-linear interaction), but drop the linear ones? The best fitting model appears to be one where only the quadratic moderation of E is retained and all other moderating paths (i.e. linear A, C, E, quadratic A and C) are dropped.

Any advice would be much appreciated! Thanks!