You have the arrow in the correct direction. The magnitude of the effect is unknowable. But sociology/psychology/etc are highly, highly susceptible to bias and political leanings.
It would also be interesting to repeat the experiment with studies from different points in time (when there was more ideological diversity) to see how the effect varies with homogeneity. My guess is that if there is an effect, it varies nonlinearly with diversity--in other words, a field with 90% political homogeneity might have an effect that is ten times that of the same field when it was only 70% homogeneous. I suspect that once you hit 90%, dissent is much more effectively suppressed than at 70%. But maybe not?
The academics will deny these hidden truths but the greater population will (and already do) exploit them.