Delusions must start somewhere.
A recent study (Emoticons in mind: An event-related potential study) convincingly demonstrates that our responses to emoticons as simple as a colon next to a parenthesis :) are similar to our responses to real human faces.
Clearly, this response has been learned. No infant is born with that response and no one anywhere had it just a few decades ago.
Our tendency to respond to :) as a face arose with its use in email and texting. This response is now a well-established “public” response to a “public” semiotic. In this context, public means “understood and shared by many people.”
A public semiotic is a sign with wide currency. It is a unit of culture and often of language itself. We can see in the case of the emoticon :) that a new sign can arise due to unique circumstances and that that sign can come to have a deep meaning for many people.
The sign :) seems quite beautiful to me because it is very simple, very easily produced, and very telling about how our minds work. If the elements of the sign are reversed (: people no longer respond to it as a face, though of course we could learn to do that if the reversed sign were used that way more frequently.
I remember the first time I saw a derivative sign ;) and wondered briefly what it meant. If you had a similar experience, you may be able to remember how such a simple sign can bloom in your mind and go from something that is unknown to something of considerable significance in just a few seconds.
That is an example of the birth of a sign, the birth of a semiotic in your mind.
When the semiotic is public, we strive to learn what other people mean by it. When it is private—that is, with a meaning known only to us—there will be other, often very significant, implications.
What would a “private sign” be like? A straightforward example might be a code we use in a diary. Such a code would have at least one visual sign whose meaning is known only to us.
Another kind of private visual sign might be a facial expression that we have come to interpret differently from other people. My guess is everyone has a good many of these. That is to say, the “idiolect” of facial expressions we each use to understand other people is at least as various as different idiolects within a spoken language.
Now add tone of voice, posture, accent, word choice, topic choice, and so on to this mix. Each of those areas of communication uses signs that can and always will be interpreted in a wide variety of ways, including private ones.
Now, consider how an individual may get lost in all this. If someone ever smiled at you as they hurt you, you may have learned to be suspicious in your interpretation of human smiles. Or you may employ your own smile in ambiguous ways.
Now consider all the signs of communication and how many possible interpretations there are. Then consider the study linked above which shows how deep our responses can be to something as trivial as the sign :).
One way we form delusions occurs when our interpretations of communicative signs become too private and/or do not correspond well with the interpretations employed by other people. The other way we form delusions occurs when our interpretations of signs does correspond well with the interpretations employed by other people, but those other people are wrong.
In “public” situations—professional, commercial, business, school, etc.—it is fairly easy to communicate well enough based on established norms. But in interpersonal communication, you can only take “established norms” so far. At some point, you will have to understand your partner and be understood by them in much greater detail than “established norms,” or public semiotics.
Here is a newspaper article on the study linked above: Happy days: Human brain now registers smiley face emoticon as real facial expression.