Here's a little lesson on assumptions.

[Note: the discussion below relates entirely to Hair Room hair, but I'm sure there are implications that go beyond that.]

I had assumed that setting the skullcap to invisible in Properties would be equivalent to leaving it visible in Properties, but making its material settings fully transparent.

Not even close.

Check out these comparison area renders. Nothing else changed, just the skullcap Properties.

Some things to draw from this:

• Making the skullcap visible but transparent will make it much easier to hide the head with the same amount of hair density set in the Hair Room. In fact, in order to make the roots look only half decent, the strong part on this hairstyle required density to the level of making the rest of the hair unrealistic, somewhat helmet-like.
• Making the skullcap invisible results in virtually zero shadowing artifacts in Hair Room hair.
• Making the skullcap visible but transparent with light emitter unchecked results in blotchy artifacts.
• Making the skullcap visible, but with "Light emitter" on also leaves artifacts, but in such a way that at least with this hair colour they pretty much look like natural shadows. (More testing required.)
• One of the takeaways is that the intuition that transparency + light emitter on is resource hungry is not correct in this case, since we can make do with much lower hair density, at the same time gaining in realism.

Maybe all this is old hat to everybody else, but it's valuable lessons to me, especially as someone who uses the hair room a lot.

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3