difference in slope between the groups G = 0 and G = 1 is constrained to be the same as the slope difference between the groups where G = 1 and G = 2) and it forces the lines for the three genotypic groups to all cross at the same point when an interaction exists, which also means that slopes and intercepts of all three lines are ordered. Accordingly, an interaction will only be accurately represented by the cross-product term when these conditions are met. Particularly concerning in the context of interpreting gene-environment interactions is that use of a cross-product term to model interactions will produce regression lines that always follow an ordered interaction pattern that maps onto an additive effect even when the true shape of the interaction in the data does not follow that pattern. It is common practice to plot the estimated regression lines to illustrate interactions; however, in many cases doing so will be misleading because it will not accurately represent the shape of the interaction in the data. If the researcher did not plot the actual data points and instead relied on a significant p-value associated with the cross-product term as evidence of interaction, this
