As far as social trends...
Based on testing reactions to a set of stock images, Beth Eck (cite at end) has suggested how nudity is often interpreted socially in the US:1) Artistic
Most typically: Images that can be read as "old" (such as black and whites) or bodies that can be read as not not fitting contemporary ideals of beautific shape.
People also tend to imagine, when asked in the abstract, that paintings and sculptures are more likely to be art than porn.2) Pornographic
First of all, "not art!" People think they "know art when they see it" and don't socially feel much need to justify it. Yet, Eck found that people tend to be less clear on how
they know art, as opposed to porn. The "marks" of porn are popularly assumed to be more obvious.
Porn is explained as "provocative" poses of nude bodies. However, what is "provocative" has changed historically: In her own time, the Mona Lisa was often thought to be staring out a challenge or invitation to so many men who passed her frame. In more contemporary measures, often female orifices are focused on by the camera/perspective. 3) Informational
E.g. the "cultural" side of National Geographic. Basically if the people and setting "looks foreign," it's okay. With the exception that there is a public debate about whether even here, young boys
might acceptably be shown in the nude. It's more accepted to depict a mother and child with nudity to convey as a nurturing if emotionally distant relationship (high infant mortality in developing countries).
Also, nudity in medical texts. Surrounded by appropriate jargon and perhaps, with signs of an operation underway.4) Commodity-marketing
(advertising)... Eck says this category is culturally "under construction."
A certain glossiness, telltale coloration and camera "tricks" are often cited in deciding that an image is being "sold" hard enough to fall into this category. Younger generations are more likely than their elders to allow that some nudes are "simply" marketing tools.Eck notes that some images, such as Demi Moore pregnant or nude pieces involved in debate about funding for the National Endowment for the Arts (particularly those that managed to go on display outside the museums) actually fall easily into multiple categories. In those cases, people usually go to great lengths "explaining" how they see certain parts as more important such that they can place the work as one of these four.
Eck, Beth A. 2001. "Nudity and Framing: Classifying Art, Pornography, Information and Ambiguity." Sociological Forum 16(4): 603-632