Alexithymia
The term "alexithymia" refers to an inability to ascertain or describe one's emotional state.
Note that this doesn't mean one is unable to feel emotions or that one is unaware of emotional turmoil or elation they might experience.
Instead, they're unable to put a label on those roilings and surges, and thus unable to communicate them to others, or consciously address them in appropriate ways, internally.
Some studies have estimated that as much as 10% of the human population experiences some type of alexithymia, though it's considered to be a spectrum rather than a black-and-white "you have it or you don't" sort of thing—so it's possible to be very mildly alexithymic, even to the point where some researchers consider aspects of modern masculine normativity (behaving in what you consider to be a stereotypically "manly" way) could be construed as mildly alexithymic, especially in regard to ignoring one's emotions or pretending they don't exist.
This trait is thought to sometimes serve as a precursor to other psychological conditions and disorders because it can disconnect us from internal data we might otherwise use to inform our assessments of self and others, and to ascertain the nature of our relationships, responses to stimuli, and so on.
The more we're disconnected from this type of emotional data—consciously, by trying to be more stereotypically "manly," or unconsciously because it's just part of our larger jumble of personality traits—the more data in our environments we might miss, and thus might not be able to work into our internal and interpersonal parsings.
Researchers have found that there's a high correlation between alexithymia and autism spectrum disorders, PTSD, anorexia nervosa, major depressive disorder, and various panic-related disorders, among others.
This could imply a relationship between these psychological states and traits, but it could also be that we lump anything emotionally deviant-seeming under a single umbrella-term, and alexithymia isn't so much an independent trait sometimes connected to other psychological concepts as it is a word we use for different things that look superficially similar: different sorts of numbness or disinterest toward emotional data-points the majority of people unconsciously collect and assess.
One of the primary (and oldest) theories about what causes alexithymia is related to the hemispheres of the brain. Basically, emotional data from one side doesn't make it over to the other side where it would typically be attached to non-emotional data for better understanding of both.
This is based on dysfunctions measured in patients over the years, and many correlated disorders also involve either miscommunication between brain-hemispheres, or damage to some neurological component involved in that communication process.
An alternative theory is that alexithymia—which is often called "disaffectation" in this context—is triggered when someone experiences emotions so overwhelming that they essentially shut down their future responses to emotions so as not to lose themselves in that kind of torrent of feeling ever again.
This latter theory, then, revolves around the concept that this is a psychological issue that could potentially be worked through, and is the consequence of psychological trauma.
The former theory, in contrast, tends to orbit around a collection of possible brain injuries or malformations: something doesn't work right in the brain as an organ, as opposed to something not working right in the psychological state emerging from that brain.
Wherever it comes from and whatever labeling system we might want to use for it, alexithymia is also generally thought to make perceiving the emotional states of others more difficult.
Not only can folks who have this trait not easily or accurately assess their own emotional state, but they have a lot of trouble empathizing and picking up on cues and understanding what someone else's state might mean in practical terms.
All of which can make relationships of any kind difficult, which is part of why there's been so much research into this concept: many people with some flavor of alexithymia end up in therapy—solo or with their partners—and that's led to a lot of interest from folks who research this kind of thing.
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