One of the curiosities about how psychosis is defined, is the fact that clinically, delusions are defined as strongly held "wrongful" beliefs that don't respond to counter-proof, and that aren't shared with others in a subculture.
In other words, believing even very bizarre conspiracy theories such as "the earth is flat" isn't a delusion, though in a broader linguistic sense it is sometimes referred to as such.
In this post I wanna talk a bit about why that is, and why I do think that it's important to have a distinction between clinical delusions that happen in psychotic illnesses, and strange beliefs that arise in other ways.
So why is it not a delusion if it's shared with a subculture?
I think the reason for this distinction is that delusions experienced by people with psychotic disorders are something that comes from within, rather than something we've been taught to believe. Per definition. Psychotic people aren't particularly "gullible", we don't lack critical thinking skills, we have an illness that make us believe random untrue crap in a way that makes us unable to think critically about it. So while a psychotic belief could be inspired by something we've come across (like a conspiracy theory) our brain is generally gonna take it as a seed and run with it. Therefore we usually quickly get out of bounds from the 'community' that might have inspired our belief anyways.
Overall, we are less prone to having gotten our beliefs from others and are more prone to being the originator of a belief. In something like folie a deux, a non-psychotic person is taught reality from a psychotic delusional person, f.ex. a child growing up with a delusional parent. The child might appear at first glance to be psychotic, but actually they only believe those things because that's what they were taught by someone they consider an authority. If you remove the child from that environment, you will usually be able to help them regain a better understanding. Similarly someone might grow up in a cult. And they are believing what they are being taught, and their parents are believing what they have been taught. And there will be most likely an originator to the cultish beliefs. That person might be maliciously making things up, or they might even be psychotic and delusional. But the people who are being taught these things as facts are behaving like most humans, as social creatures who's reality is defined by their context.
Most people's context is defined along the lines of consensus reality, but if your social context is not aligned with the majority consensus reality, you are still aligned with the beliefs of your social context if you share your weird beliefs with a subculture. Your brain didn't independently come up with a wild belief that is out of touch with everything you know/have been taught.
Consensus reality is a consensus. And even if the consensus you follow is shared by only 2% of the population, if that 2% is all the people you relate to and consider to be the people "in the know", then you are in a way not going against your contextual consensus reality. You've just picked a less popular one.
So what defines a clinical (psychotic) delusion is that it does not align with any consensus about reality that you have access to. It's your own, and it's unlikely that you have allies who are supporting your beliefs. Though in rare cases a clinically delusional person may be contributing new material to a subculture, that others then start believing, and as a result they do share their beliefs with a subculture. But they didn't just learn the belief from the subculture, the belief is growing and morphing independent of the group.
But yeah that's all clinically speaking. In a broader linguistic sense, I think people use "delusional" to refer to anyone who has beliefs that aren't aligned with the majority-consensus-reality, or even more simplistically, that aren't aligned with the speaker's understanding of consensus reality (usually as an insult). So an atheist might refer to the religious as delusional, and vice versa.
It may be a losing battle to get wider society to stop using 'delusional' in this way, but I think it is at least helpful to talk about how such "delusions" differ fundamentally from the psychotic experience.
