I'm running a regression model to see if my dichotomous independent variable (family type) predicts my outcome variable (well-being). I'm including four mediator variables, as a parallel mediation. I'm using Hayes Process Macro Model 4 for this.

But...the problem is that I know that two of my four mediators have a direct curvilinear effect on the outcome variable. The other two have a linear effect on the outcome variable.

How would I go about this? I've read that if I treat it all as linear, I could get a false positive because of the curvilinear. Ideally, I'd like to not split the four mediators into four models because I'm running it across four different outcomes and two comparison countries...so I'd end up with something ridiculous like 32 models (4x4x2) instead of 8, so parallel mediation would be more concise.

Thank you so much!


1 Answer 1


You can use a spline of the mediator, if you don't know the shape of the relationship, or the mediator squared, if you think it's quadratic, or some other function of it.

I don't know SPSS at all (which is in your tags) but I know you can do this in R or SAS.

  • $\begingroup$ Okay - this is very helpful and not as complicated as I was fearing, thank you!! It's quadratic - so in theory I'd just run the regression as normal, but using the two squared mediators in the place of the two curvilinear ones? $\endgroup$
    – Hannah
    Commented Aug 1, 2023 at 17:02

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