Parahuman

Parahumans are humans who have undergone a trigger event and developed superpowers. The slang/colloquial term for parahumans is "cape," which is typically (but not always) used to refer to parahumans who wear costumes, but may also refer to Rogues.

Biology
Parahumans and potential parahumans have a structure called the Corona Pollentia in their brain. The size, shape, and location of this structure varies with the individual parahuman, but tends to be located between the frontal and parietal lobe. The Corona Pollentia, along with a substructure inside the Pollentia called the Gemma, allows the parahuman to control their abilities.

The Corona Pollentia starts out at about the size of a golf ball. During a trigger event, the Corona Pollentia activates in a spurt of growth, spreading further into the brain and forming a connection with the parahuman's shard. It also seems that parahumans can pass on a similar power to their children without them having to suffer severe mental trauma. It is important to note that while children of parahumans are more likely to gain powers, it is pointed out that the trend is more likely to do with long-term exposure to parahumans than genetics.

Trigger events can radically alter a parahumans biology, as seen with certain Brutes and Changers, as well as having other effects such as reducing or even removing the need to sleep or obviating the need for a functional body or brain. Another example is that parahumans with sonic powers tend to have altered voices.

Pre-Gold Morning powers were also able to alter themselves due to permanent changes in anatomy or biology of hosts, be it mutilation or deeper alterations, as exemplified by Victoria Dallon, Valefor, Bonesaw's projects and The Unmasked.

Psychology
Parahumans are naturally inclined toward conflict, as shards choose humans that are predisposed to using their powers. Additionally, shards subtly nudge their hosts toward conflict, playing off of their existing neuroses and mental condition. Parahumans tend to be more emotionally volatile, their emotional peaks and valleys more exaggerated, their trigger issues causing bigger eruptions. This emotional volatility exacerbated conflicts and made gathering large teams of parahumans difficult. Parahumans who were isolated and not regularly challenged had a mild tendency toward increased mental abnormality, becoming more paranoid and aggressive.

Parahumans, especially those who trigger at a young age, can have their personality altered by their shards. The more conflict parahumans get into, the firmer a "foothold" the shard has to rewrite their consciousness and subconsciousness. In extreme cases, the shard can increase impulsiveness, cause obsession, amplify neuroses, exaggerate personality traits to the extreme, or overwrite parts of the mind. In lesser cases, the influence is subtler, with the shard tapping into the emotions and base impulses of its host in order to encourage them to use their power in a combat situation. In some such cases, parahumans become restless or depressed while out of battle, the feeling only fading when out and fighting. In other cases, the parahuman is actively rewarded by the effective use of their powers, instilling a desire to use them more often.Depends on the shard. Bonesaw elaborates on the idea by noting 'breadth and depth' in her interlude. If the shard gets you while you're young, it can shape your personality across the board, on a deeper level. The more conflict you're involved in, the more toeholds it gets to rewrite your consciousness and your subconscious. To alter your thinking, it needs to do it as a part of the trigger event, or as part of the brain's development. In the extreme cases, the shard can leave you with an impulse (Must fight when a fight presents itself), help set up an obsession ("Wall myself in!"), steer a neurosis in one particular direction (specific hallucinations rather than random ones, of you hurting people, pushing someone down the stairs, etc), create a link between A and B (Being around fire makes subject lose empathy and inhibitions. With lower empathy and inhibitions, subject uses power to make more fire.), or steer a personality trait to an extreme (Must be on top, I answer to no one!), or they just overwrite stuff (Can't understand humans, only dogs). In the lesser cases, it can be a nudge, hard to distinguish from one's own psychology. You might be on the fence about something, trying to make a call, and the passenger pushes you one way over the other, based on your own feelings of doubt or fear. It might tap into emotions, and dampen X emotion while promoting Y, just dampen them across the board, or take the joy out of day to day living while adding excitement to the cape life. A vague sort of depression that only goes away when one's out and fighting. Sometimes, as mentioned before, it's set up as a trap, a flood of emotion or a set of mental switches that get thrown when a prerequisite is met - such as a cape just steering clear of all confrontations, except the shard set it up so they can't, and they have a sort of limit break/command cutting in that mandates them to fight in one way or another. Or it plays off a limit or a berserk button that already exists - Damsel can't spend too long being anything less than top dog or she gets restless, and if she goes too long despite that, then she has to act, she's acting without thinking about it. This takes time and effort for the passenger, and a host that doesn't demand that time and effort (by circumstance or intent) is going to develop a better connection with the power. This in turn is a reward of sorts. If Damsel did kill the local capes and assume control over the area, fighting off all comers, she'd find her facility and control with her power just ramped up like crazy. It varies from cape to cape and shard to shard, and it varies depending on the host, the host's background and the host's personality. Beyond that, other influences include the passenger playing fast and loose with the power itself, as it controls the metadata, which may be more visible if the subject breaks from their norm in terms of consciousness (gets a concussion, tranquilized), working off base instincts and impulses like 'stay camouflaged' (be a little more creepy and unsettling), intimidate/dominate (passenger works behind the scenes to make you look a little more dangerous as you mutate/grow/surround yourself in the aura of your power), etc, etc. In more pronounced cases, the power is just plain controlled by the passenger, not the host, and the passenger makes the seemingly random or uncontrolled aspects generate more conflict... pushing a power to kill rather than leave someone alive, or a thinker power turns up a vision of something the subject didn't want to see. On the macro level, too, don't discount the fact that some shards (particularly powerful ones that warranted attention) are just sent to specific people, with the idea that it's a combination that's going to promote more conflict just by the sheer dynamic of it (Powerful person with a destructive power, a desperate person with a power with negative implications). - Wildbow on SpaceBattles

Some psychological commonalities exist when dealing with parahumans with a specific type of power. Masters, for example, have a systematic tendency to have interpersonal problems, although this is true to an extent of all parahumans. As a general rule, Breakers, Movers and Trumps tend to feel distant from the rest of humanity. Trumps become focused on parahuman powers, Breakers separate themselves from others, and Movers avoid confrontation and have trouble feeling rooted and staying in one place. Shakers tend to focus on context and their environment, in a need to feel secure. Changers worry about not being able to change back. Tinkers, based on those seen in the storyline tend to have their personalities influenced by their gender. They also have to deal with seeing the world through the lens of their ability to create i.e. everything looks like potential resources given the right tinker.

Despite the prevalent influence of shards, parahumans are independent beings and act of their own accord. Shards merely influence their cognition on an instinctual covert level, with overt control of a parahuman by their shard being very rare.

Population
It is implied that there are a great many capes in the world, but that they still form only a small percentage of the population. Brockton Bay has roughly seventy parahumans that have been introduced and described in the course of the story, implying that capes comprise less than one percent of the population.

It is also mentioned that there is about 1 parahuman for every 8000 in urban areas.

Culture
Many parahumans follow a certain code of conduct that exists in different forms around the world. It is a general set of "rules" that parahumans use to prevent situations from escalating out of control. A parahuman's civilian identity is considered sacrosanct, with the act of unmasking a cape usually considered a grievous offense.

"Cape names," the name assigned to a parahuman's costumed identity, are a big part of parahuman culture. Not stealing another parahuman's cape name is considered common courtesy. Villains have been known to hold outright duels over the matter, while heroes go to great lengths to secure permission to take a name. Cape names that are considered "stupid" or offensive will be contested, with the community striving to rename the cape in question.

Background
Parahumans started to emerge around 1982, if one counts Scion as such, with the first true "heroes" appearing in 1987.

With the propensity for parahumans to appear in marginalized groups, political power shifted.

The PRT was formed in the United States to integrate parahumans into existing society, among other reasons.

Story Start
Society had largely accepted parahumans as a necessary evil to combat things like the Endbringers.

Gold Morning
Cauldrons purpose was revealed when Scion, now revealed as Zion began attacking Humanity. On a multiversal scale.

Post-Gold Morning
A full understanding of parahuman biology became known to the general public. The idea of "passengers" granting powers became commonplace.

Trivia

 * The term parahuman is constructed out of the prefix "para-," meaning abnormal, and human.
 * What a general civilian knows about parahumans is wildly individualistic.