The Manton Effect is a common tendency among powers to fail to affect living targets, first discovered 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.
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 costumes destruction. This is due to the Agent reading the hair as still "part" of the parahuman
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. Allowing 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."
This article contains MAJOR SPOILERS relating to
some of the most pivotal parts of Worm. Although all articles contain spoilers, this article will reveal major plot twists and should almost certainly be avoided unless you have finished the story.
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 the origin of the common tendency for parahumans to have a minor Breaker ability protecting them, and it's misfiring in a miscalibrated shard is responsible for Case 53 mutations.
↑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
↑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)
↑As Tristan glanced at me, I made a rectangle with my fingers. “It’s complicated. It fluctuates. I could name some terms and things that apply there, but I don’t want to bore you.”
“I’ve been bored enough times. I know a lot of the terms. Sechen ranges?”
“That’s one of them. Powers often get stronger with certain influencing factors. You read up on that?”
“We did a ton of testing with Reach, and we saw a lot of parahuman science people while we were trying to figure out a solution. They think it’s a straight multiplier. I have one point three to one point six times the strength and overall fitness.”
“Handy,” I said. “And Byron?”
“A bit of resistance to temperature extremes. He gets a higher percent, he’d probably remember his specific numbers better than I do, but unless it’s winter or we’re dealing with a heat wave, it doesn’t apply as often.” - Excerpt from Shade 4.5
↑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]