There is a type of formal mental illness called Borderline Personality Disorder. It has also been called Borderline Personality Organization when it was still viewed as an emotional developmental disorder. In that framing, it was supposed to reflect an emotional development of about a two-year old, though the intellect might develop normally. Poor parenting, creating an inability of the child, usually female, to bond and identify properly was believed to be the cause. The name comes from another way of looking at the disorder, of neurotic defense mechanisms trying to deal with psychotic forces but coming up short.
There is now a lot of controversy about how the condition occurs, and it will not surprise you that I lean toward the newer evidence that there is something genetic about it. The people who have the disorder have difficult, usually unhappy lives. There are treatments, but they are long and involve a lot of picking yourself up after having fallen down. They are often disliked by the people around them, including the professionals who treat them, because their condition causes them to act in emotionally disruptive ways. There are a few professionals who like working with them and don't find them so frustrating - God bless them.
Something that has changed recently is that social media gives them much greater power and influence than they had when they could only touch a small circle of people. In the last decade everyone seems to be weighing in on how Facebook, Twitter, Snapchat, wide availability of comment sections,and other social media are bad for us. They give us a great deal of entertainment, information exchange, and opportunity for the common person to have a voice and some influence, but we are increasingly seeing the costs: increased depression, anxiety, bullying, doxxing, depersonalisation, anger, deteriorated interaction and debate, and a dozen other problems are embedding themselves so deeply in the rising generations that they have become permanent parts of their culture and will have some lasting impact whatever we do from here.
It isn't often mentioned, but a great deal deal of this is driven by the new outsize influence that people with Borderline Personality Disorder have over all of us because of how they can portray their reactions as common to many others, and create the impression of a whole culture angry with someone and ganging up on them. They often do not have an impression of themselves as people with a disorder, and thus sincerely believe their angers and reactions are common to humankind - that they are things that other people should feel. This impression is very strong in them.
Here are some characteristics. See if you can see how they put their thumbs on the scale of the public discourse. Note: I am referring to the more intense versions of the disorder that one sees in a hospital. There are many out there who are less severe, who have the tendencies I list but have learned to manage them. They deserve credit. It's a hard life, being in a continuing state of emotional sunburn, so that small discomforts become magnified.
Feelings are facts. How they feel this moment is the most important fact in all their actions. (Treatment focuses on getting distance and objectivity.) Their feelings are more powerful than others, like a teenager's. If they feel threatened, then the other person is threatening. If they feel that someone hates them, then they regard that as true, and often expand it to believing that the person hates women in general. If they feel put down, they equate this with assault or rape.
Their feelings can change rapidly. Then can quickly become enraged and self-destructive, but when they have been distracted to something new, they can be bright and excited very quickly as well. If they were angry or suicidal an hour ago, so you don't want to decrease their supervision now they will become irate because they know they are just fine, and are angry at you for not seeing that. Because they feel just fine in the moment.
There is the common statement "You aren't listening to me!" They cannot comprehend the idea that someone could understand what their point is but not agree with them. They understand intellectually that such things are possible, of course. They aren't stupid. But in the moment, they believe that if you really, really understood what they were saying you would have to agree with them. You don't agree with them, therefore you have not been listening closely enough. When I have said "I think I am listening, Amber. You think that the clinician in the Emergency Room was rude to you and jumped to conclusions. I just think there could be another side to that story" they tend to find it infuriating. They cannot bear to even hear it. If they cannot cast me out from their presence, they walk away from mine.
They get along better with pets, and will volunteer at animal-places. They lead the world in unofficial support dogs.
People either either all-good or all-bad to them. Again, they intellectually and theoretically know that humans are mixed, but they perceive them in one category or the other. They can thus easily feel betrayed and enraged.
They are more likely to have been abused as children, especially sexually. They are more likely to be gay, lesbian, or bisexual. They are rather pansexual, really. I don't have enough personal experience to verify this, but I am told that there is an increased percentage of trans people among those with the disorder. My small sample size would tend to confirm this. I have not met anyone who says it is over 50%, however, so don't overgeneralise this.
I will add other things as they occur to me.