I think you are completely misrepresenting the literature in the field. There has been decades of research on inner monologues, but whether anyone truly has no inner monologue is still a matter of debate, and suggesting that it could be as much as 50% is absolutely wild.

One recent example is Nedergaard and Lupyan (2024), who used questionnaires on 1,037 participants and found no one who reported a complete lack of inner speech. They did show a link between lower frequency of internal speech and lower performance on sole verbal cognitive tasks.

But this was frequently misreported in popular science news, which may be where you got the idea. For example, Science Daily's headline "People without an inner voice have poorer verbal memory" and subheading "Between 5-10 per cent of the population do not experience an inner voice" certainly make some bold claims (although still well below your "up to 50%" statistic). But just a few lines into the article it's been rephrase as "between 5-10 per cent of the population do not have the same experience of an inner voice". This is more accurate, as all studies agree that there is a variety of experiences of inner voices / monologues, but a different experience is not the same as an absence.

In another comment you make reference to the experience sampling study (where a buzzer would sound and participants would record whether they were experiencing an inner monologue) which I assume is the work of Heavey and Hurlburt. It's true that they claim that 5 of their 30 participants recorded no instances of inner voice, but let's be clear about what the experimental procedure was: the participant would turn on the buzzer, which would buzz at a random time (an average of every 30 minutes) and the study was based on two periods of five samples. So, ten data points collected over approx five hours.

Even people with strong inner monologues report different frequencies of inner speech depending on their activities. Many people do not experience inner speech when actively engaging in other verbal activity - talking with friends, watching a video; while quiet focused activities such as golf show much higher reporting of inner speech. So the absence for five individuals of any inner speech during those ten particular samples is in no sense equievlant to "16% of peole have no inner monologue". Indeed even the study's authors acknowledge "it is possible that these participants may all have actually had quite similar inner experiences; it is merely the reports of those experiences that differed."

Tldr: I think you're making some very wild claims about this subject, without posting sources. No significant study I know of claims that any sizable percentage of the population have no inner voice, (although there certainly is an interesting variety in how frequent and clearly it is experienced.)

Internal monologue is entirely a subjective experience, and I don’t think there’s any other way to study it than by asking people. Just because someone isn’t consciously aware of it doesn’t mean it’s not there. Just like if we asked people whether they have a blind spot in their visual field, everyone would say no - and everyone would be wrong.