The Manton Effect is a common tendency among powers to affect either living targets or inanimate objects, but not both. This seems to be a manifestation of a more general tendency of powers to be limited to facilitate their use by humans; more specifically, powers tend to have "in-built" controls to avoid harming the parahuman using them. This phenomenon was first recognized and codified by William Manton.
All powers come with certain limitations - they might only target certain materials, have a limited range, limited precision, and so on. For most (but not all) powers, their range of effect stops at the outside of a person or animal’s body. Many of the exceptions are powers that only work on living creatures, such as certain Masters like Nilbog, or Panacea's biokinesis.
This effect severely reduces the potential lethality of most powers; most telekinetics can't reach into your chest and crush your heart; most people who can create forcefields can’t create one through the middle of your body and cut you in two; most pyrokinetics can't generate fire inside your brain to instantly boil you, and so on.
For most of the story, the reasons for the Manton Effect are unknown, though the protective tendency of the Effect is well recognized.
A common exploitation of the Manton effect is to have a costume made, whole or in part, out of the subject's hair to prevent the costume's destruction. This is due to the Agent reading the hair as still "part" of the parahuman
Powers will ignore walls before they ignore a floor. This is the reason intangibles do not sink into the ground.
Vista has the ability to stretch and compress space, but this becomes a great deal slower and requires more effort when the space she is working with is occupied by people, and she is unable to stretch, distort or compress a living individual.
The author notes in the comments of Interlude 5 that this is due to Vista's ability involving a great many small, interconnected events through the space she's affecting, and the Manton Effect prevents said effects from occurring inside people, forcing her to work around them.
Bakuda, by contrast, researched Vista's powers and found a way to create a controlled distortion of flesh, and specifically references the Manton Effect when explaining this.
Faultline can cut through objects with a touch, but can't cut through living things - even plants.
Sundancer is never burned when she summons her eponymous sun. Instead, temperature completely normalizes in a set distance around her.
Crusader has a favorable Manton limitation. His ghost duplicates are unrestrained by inanimate objects but will interact through living objects; this allows them to stab through armor. These replicas could also lift him into the air, giving him a limited form of flight.
The most common way for a parahuman to bypass the Manton Effect is to undergo a second Trigger Event.
Faultline surmises that the Manton Effect might be a mental block set in place when an individual gets their powers, protecting them from hurting themselves with their abilities, which is overgeneralized to include all living things rather than just the user. Her attempts to retrain and circumvent this mental block, however, prove futile.
Bakuda was able to partially bypass the Manton Effect when she created a bomb based on Vista's abilities, which are normally Manton-limited.
"I won't spoil the conclusion if you're eager to see this through."
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The Manton Effect is a result of a person's shard imposing limitations to ensure they don't accidentally hurt themself. During a second Trigger Event, the shard can refine its technique to only protect the host.
This desire to protect the host is also why there is a tendency for parahumans to have a minor Breaker ability protecting them. Host protection measures misfiring in a miscalibrated shard is responsible for Case 53 mutations.
A hard-coded Manton Limit common to most powers is that they do not commonly operate in Space. This is partly to prevent Parahumans from moving so far away from their Shard that transmitting power to them becomes a hassle, and partly to prevent the host species from using their powers to escape the planet.
↑Fluff: Rereading another horrific unintended consequence of someone messing about with superpowers strikes me:
If Shatterbird can cause sand and computer chips to explode like that, she will also have destroyed a very large number of the city’s plants due to several families of plants having internal silica dioxide structures called Phytoliths in their leaves and stems.
Especially vulnerable are: Palms, Sunflowers, Forgot-me-nots, Squashes and melons, nearly all ferns, Magnolias, Orchids, Elms, Nettles, Ginger and related herbs, and most importantly every single cereal and grass!
All of the above will have been turned to watery goo by silicokinesis powerful enough to send up sand mushroom clouds. Every farmers field round the city is now sludge, and every blade of grass in the parks is dead.
If Shatterbird wants to bring about the end of civilisation, a road trip through the corn belt would work wonders! Wildbow: Manton effect isn’t necessarily limited to just people. - Comment by Wildbow on Plague 12.6
↑The second theory was that it was the Manton effect, that broad-as-bells term for the built in protections and limitations of the power. The theory was that the built-in protections of the power only protected what I saw as a part of me, and it had taken some time before the costume was that much a part of my identity. - Excerpt from Daybreak 1.7
↑Behemoth would be an obvious exception to this last rule.
↑◈ Deness - Girl, 15, so shy she barely speaks to her family, let alone you. Her smoke disintegrates material, including her clothing. She recently cut off her hair to use it to fashion a fairly skimpy costume that won’t disintegrate, and got tattooed from the neck down, with tattoos aimed at providing some illusion of modesty, which gets your attention as a red blooded boy. - The Fallen, for Fallen Son
↑“Okay, so… I have a bit of a crazy idea,[...]Hair,” I said.
“No,” was the response, without a beat missed.
“I can’t promise it would work, but hair can confuse the Manton effect. It might be that the power gets confused because it’s a part of your identity and a part of you, but it’s not alive either. There are parahumans who impregnate their costumes with hair to make them resistant to their own powers. There are some who have costumes that are just hair, or mostly hair, but those are pretty scanty, as you can probably imagine.”
“I think I’ve heard of that parahuman,” Sveta said. When I arched an eyebrow, she said, “The hair impregnation thing.” [...] “How much would I need?” Ashley asked.
“If it did work, you might not even need much. A strand every quarter-inch or so, along the length, or along the parts that are likely to get clipped by your power. Maybe a bit more.” - Excerpt from Shade 4.6
↑"Or the floor. You said we have to watch the floor,” Parian said.
“Yeah, but there isn’t much we can do about that,” I said. “The way powers tend to prioritize things, walls will be a problem before floors are. [...] PRT research. Classes. There’s a whole mess of research into why people like Shadow Stalker from our hometown didn’t fall through the floor to the planet’s core.” - Excerpt from Blinding 11.6
↑Taliesinskye and Psychogecko are pretty on target. Though Kaiser’s power wouldn’t prevent him from growing armor -on- someone (which is essentially what he did when he trapped Lung in the pyramid of blades, only it was a more offensive use).
The Manton effect essentially says that for most capes that does something at point X, or originates at point X, that point X can’t be inside another person. Different capes are affected by this to different degrees or not at all.
Capes like Vista and Faultline are extreme cases of capes who are affected a great deal; Vista’s power affects an area, and it’s exponentially harder to use if there’s more people inside that area. This is mostly because her power is actually lots of little interconnected events, some of which are bound to fall inside people in the area. Faultline’s drawback is that she simply can’t affect another living thing with her power, period, likely because she’s extending her power into whoever or whatever she’s touching to sever molecular bonds and ‘cut’ them.
On the flip side of the coin, for capes with powers that wouldn’t work if they couldn’t reach inside other living things, the Manton effect doesn’t usually apply. Taylor’s one such case. If the Manton effect was as severe in her case as it was for Faultline, she wouldn’t be able to extend her power to the bugs’ minds (such as they are) to control them or get intimate details on their biology and locations… so she wouldn’t have a power at all. Panacea and Regent are other examples of this at work.
In the end, though, scholars in the setting haven’t fully researched and understood the Manton effect and why it exists. So the fact that there’s some confusion on the matter (to the point we may be talking about different effects that are all being (erroneously?) gathered under the same umbrella) is perfectly ok. - Comment by Wildbow in Interlude 5
↑There’s other limitations or advantages that come with the powers. Sundancer over there can’t be burned. Temperature completely and one hundred percent normalizes within a certain range of her body. - Excerpt from Scourge 19.3
↑Justin Crusader Can create shades of himself to fight, lift himself. Favorably manton limited as shades ignore inorganic matter, penetrating armor and passing through walls. E88/The Pure Master 6 - parahumanList, bolded edit by Wildbow.
↑Crusader – One of Brockton Bay’s white supremacists, Crusader wears armor and carries a spear. He can create ghostly duplicates of himself and his possessions, which pass through inorganic objects, but not organic ones. - Cast (spoiler free)
↑Crusader, Justin – Can create ghostly replicas of himself that can float and can pass through body armor and walls. These replicas can lift him into the air, giving him a limited form of flight. - Cast (in depth)
↑Rune – A powerful telekinetic and white supremacist, she can move nonliving objects she touches, each one potentially weighing several tons. Dresses up as a wizard. - Cast (Spoiler Free)
↑It was reassuring to see the only other person left in existence that understood, that she couldn’t hurt with her power, should a freak accident happen. -  from Eclipse x.8
↑Broken shards don't care so much, some powers can draw energy in other ways, but by and large, powers stop being responsive or start getting fucky at some point between the upper atmosphere and 400k km out. Many powers are manton limited so they don't actually get out into the vacuum. - Elaboration on Reddit