The "IKEA Effect" was not designed as such, it was stumbled on accidentally. The reason IKEA furniture is assembly-required is the same reason that your monitor stand comes detached from the monitor: because these companies operate on the principle that their products be flatpackable (minimize shipping dimensions).
Giving a theory a name doesn't mean it's a real effect. I would like to see a blind AB study that shows the effect is actually true and not someone's pet theory but which is actually false. I agree that the theory is nice and has explanatory power. Let's do a controlled study to see if it is real.
I wonder if you would like to see an AB test though? I haven't read any of the other comments in this thread yet, so stop me if you already answered that.
You also missed my point that IKEA effect is not necessarily a "gratuitous" assembly step, just any step(s) at all.
