Let's consider a couple ways to photograph people.
Consider the standard, what, about five light setup? Main, fill, hair, catchlight, background? Something like that. People look at these and say "how natural" when they are anything but. We do not after all live on Barsoom. This sort of setup does a bunch of technical stuff. I'm not an expert, but I think among the desirable things are: no deep detail-burying shadows, but adequate modeling for a nicely round look; good separation of the subject from the background; a nice lively twinkle in the eye.
But there's another thing you get out of that, a level removed. It looks done, it looks expensive, complete, finished. Well done, it feels like one of those living rooms with the cream-colored leather couch and the dried cattails in the perfect ceramic thingy in the corner, and the vase filled with balled up paper in just the right colors. Done. Designed and executed to all flow and fit together expensively, perfectly.
When the CEO has a good portrait done, the photographer will ideally get a beautiful perfect moment out of him, and then will insert it into this perfectly polished photographic trope, and it will look expensive and polished. Just the sort of image you want your CEO to project, most of the time.
And it is a deeply photographic idiom. Painters can accomplish all the technical details without introducing light sources - photographers cannot, because modifying light is pretty much the only thing we can do. Photo-graphy, light-drawing, that whole deal, right? So we end up with a pile of lights.
Consider another current approach to photographs of people. Dead-on flash, or perhaps slightly off-center. Yes yes Terry Richardson is an asshole. There's lots of people running around out there doing slight variations on the theme. In terms of all that technical niceness, the modeling, etc, it's a total bust. But it's still a deeply photographic look, one that arose out of the vernacular. These things are totally photos that look like photos and not even a little bit like anything else.
What's this look communicating?
Probably usually something about spontaneity? Unposed, natural, in the moment, hip, active? Stuff like this.
In both cases the associations we have with the style (polished/"done"/expensive, spontaneous/hip/natural) derive from the history we have with these pictures. We've seen a lot of million-light portraits, and we associate them with a certain kind of thing, where the subject is carefully posed and a great deal of care is being taken. We've seen a lot of on-camera-flash snapshots, and we associate them with spontaneity.
In both cases, photography has built its own set of associations. There's little or nothing "fundamental" about these things, it's simply that there's a long history, now, of photographs. We're steeped in them. We associate certain looks with certain things.
And thus the connection to Sally Mann and her dismal wet plate photographs, which look the way they do not through anything fundamental about vision or psychology, but largely because we know what old photos look like because we've seen some, and these look like old photos.
This, in turn, leads me to wonder how much of what we get out of a photograph is fundamental in the way our eyes and brains work, and how much is simply because we've seen a lot of photographs? There is a whole school of thought that wants to explain it all in terms of neurology and brain science. Eye leading, rules of composition, etc, etc. But it's clear, at least to me, that at least some of this stuff is learned simply by having a lot of photos thrust in front of our eyes.
Musical theorists used to (and I dare say still do) drone on about whatever the current theories of tonality are, and how they're deeply wired into the human brain. Except, apparently, for all those blokes in Africa and Asia who find western ideas of tonality weird. Turns out that practically all this stuff is learned, it's essentially a local, social, phenomenon.
This has worked out extremely well for music, because there is apparently always somewhere new to go. What has been learned can be unlearned, can be tweaked, can be built upon. If Bach's ideas of tonality had indeed proved to be hard-wired, the final word, music would be a lot less interesting today.
In some areas, it's working out OK for photography, too. Some of the most influential and important work being done today is blessedly "bad" photography being done by people who think, somewhere deep down, that their audience might be able to learn some new ways of seeing without being too harmed by the experience.