Thoughts are nebulous and personal, language is ordered and shared.

Your brain doesn't produce thoughts one at a time in well-formed sequences with clear meanings. Many things are going on at the same time, all mixing together. Sensory input is processed all the time, triggering anything from memories and emotions to instincts, learned behaviors and thoughts. Before a conscious thought is fully formed, there might be dozens of half-formed ones, with all but one rejected.
To add to this complexity, thoughts are shaped by the individual's life experience, memories, preferences, even the inclinations of their particular brain type. A visual thinker might have most thoughts as images or movies, that would be difficult to interpret for a pure verbal or mathematical thinker.

All of this means that until a person decides to cast their thoughts into language, it is difficult to interpret what they are thinking, even when intentionally sending the message. The brain just can't help adding all kinds of context and related stuff, that's how neurons work.
Even when the person is speaking out loud, a purely telepathic listener has to filter out all kinds of mental noise. That works for simple thoughts, but for longer conversations or complex ideas, it is much less error-prone and less tiring to let the speaker do all the filtering, ordering and converting to spoken language.

Factors that make telepathic communication easier will be similar types of thinking, shared experiences and general familiarity with the person speaking.
Family members might be able to communicate quite a lot telepathically, and the same might go for people that have worked together intensively for a long time. Strangers would find it very awkward.

It uses lots of energy

Have using telepathy use quite a lot of energy, compared to the casual speaking with each other through normal means. Imagine the process of sending some data (e.g. words) as a very complex one, based on multiple subprocesses:

• Step 1: Transform what you want to transmit into a fitting form. You could imagine it to be similar to the process of converting Text to its corresponding bytes and bits


• Step 2: Transfer the data you just created. As you want the transport to cause as less loss as possible, you want to use short (radio) waves, which require more energy


• Step 3: Convert the received data back to what they were before packaging and interpret them

All in all the energy required to perform telepathy is quite big, and it multiplies when you try to talk this way not to only one, but rather a group of people. So it is way easier to just talk normally.

Edit: An addition based on @Nuclear Wangs comment

To better visualize the concept of the en- and decoding and the fact, that you get better at it when you use it more / get older, think of it as translating into and from a foreign and at the start almost unknown language. The more you use it, the less effort it takes to encode your thoughts into this "language" and vice versa, so you can achieve the same with less energy being used / wasted.

It requires concentration

Have you heard the phrases, "think before you speak" and "speak without thinking?" Those phrases exist because spoken language is trivial. Telepathic language requires substantial concentration. Curious consequence, it also means you're more easily distracted while trying to convey a message.

You also need to concentrate to hear it. Otherwise it falls into the background noise of life (see the volume issue I present next). Coming or going, it requires far more concentration than speaking verbally.

You can't shout, you can only direct

The wonderful thing about spoken language is that you can pick familiar voices out of a crowd, which means a person shouting in a crowd is more easily heard by those who recognize their voice. You have the same kind of recognition with telepathy, but all telepathy occurs as the same "volume." This means you can't shout to be heard, you must focus your message much more accurately. Curious consequence, this means telepathy is much less useful than spoken language when it comes to searching a forest for a lost child.

A way of manifesting this idea simplistically is that you can only communicate with someone you can directly see. No binoculars or other amplifying tools allowed. If you can't clearly see the target of your thoughts, they can't hear you.

It's like learning another language: and everyone speaks their own language

Most people think telepathy is like verbal communication: you can "hear someone speaking" without your ears. In reality, telepathy has everything to do with how you think, and the majority of our thoughts aren't words. They're emotions and reactions and associations and all kinds of things that frequently don't resolve into words. It can be thought of as an entirely different language with a few borrowed words from your native spoken language and a few more from whatever other languages you speak (like Pascal, C++, Fortran, and COBOL... wait 'til those thoughts show up!)

The consequence is that until you learn how someone thinks, you're "hearing" a foreign language — the equivalent of spoken sound that has no meaning until you've learned the grammar and vocabulary. It's theoretically possible to "speak" in the thoughts of another person (a rare and very difficult skill, and very personal) but much more common to eventually learn how to understand what they "say." Curious consequence: this suggests telepathy can be readily used as a covert language.

You're constantly having to protect yourself from the barrage

One of the biggest problems with telepathy is that you're living constantly in a crowded room. You can't simply don some noise-cancelling earphones, you must train yourself to push aside the mental noise. This effort is, itself, distracting and substantially limits how much real communication can occur (similar to concentration, mentioned above). In effect, it's like conducting every conversation in a busy night club. You're constantly fighting to hear and be heard. Curious consequence: this makes telepaths prone to nasty headaches.

Let's ramp things up a bit...

Normal people are afraid...

People are scared to death of both the advantages covert telepathic communication gives the gifted and the threat it represents against both society and the individual.

So they invented jammers.

Telepathy isn't magic. It's an honest-to-goodness scientific method of communication involving dark-matter sub-atomic particles named "Ondions" that vibrate in a mental field. Research suggests that Ondions are solely responsible for rational thought, for sapient life, for the soul, if you wish to express it that way. We do not think because we are, we think because Ondions are densely interwoven throughout neural tissue, giving the brain that magic oomph that allows for conscious thought.

And what's really cool is that Ondions are everywhere, and thought is thereby transmitted in a manner similar to radio waves. Ondions "vibrate" (if this concept makes sense, it's not a physical vibration but a state-vibration. It's as entangled as any other headache involving sub-atomic particles), meaning thought creates tremors in the Ondion-quanta surrounding us and it weakens as it propagates from the center of thought by the inverse-cube — which was expected, that being a common aspect of nature.

What trained people realized was, when within the interrupted quanta of a thought-generator, their own minds can sympathize with that tremor, allowing the two thought centers to synchronize momentarily. The brain interprets this as communication (but the possibilities are far more impressive than that).

Which was all nice and shiny until a means of measuring the Ondion field and its vibrations was published in the IEEE Journal of Communications and Networks, including a description of the devices invented to measure the Ondion quanta. An organization calling itself "FrEMT" (pronounced "fr-eh-mt," for "Free Exercise of Mind and Thought") then turned the technology into a jamming tech that can block all telepathic communication.

Then they learned that they could stop a person from thinking. When all the Ondions stop vibrating in the human brain, the human... stops. It's like the soul leaves the body and all that's left is the biological organism. It makes vegetables look smart.

And then they discovered that by modulating the Ondionic vibrations they could artificially broadcast an idea...

Reuters August 8, 2022

Tragedy struck Philadelphia's 30th street station today when hundreds of people waiting for the arrival of celebrated telepathic composer André Quittet suddenly and without warning simultaneously stepped onto the tracks before the decelerating train. Witnesses were traumatized by what they consistently describe as the eerie and silent way the victims simply "stepped forward." Investigators have been unable to ascertain any motive behind the apparent mass suicide and no connection between any of the victims other than their interest in Quittet.

Quittet, an accredited master of the emerging art of érzékek, known as "telepathic music" by most people, has always been a strong advocate of …

I'm gonna go the opposite direction from some of the answers: it's not commonly used because it's too darn easy (and instant) and once you've learned how to allow contact to a given person (or people) the connection is partially retained, meaning they can more easily get in touch with you this way next time, possibly with little warning or options.

And the huge attendant issue is the effort involved to maintain both privacy (how do you keep your mind organised - you have "public" spaces and "private" spaces? No? Then if you could allow access, think about how insanely freeform and intrusive that would be) and appropriateness / germainity of what you share - you think people have trouble staying on topic verbally? What if they could dart off-topic instantly without meaning to, possibly not even being initially aware they were dragging their telepathic conversation partner along with them on their tangent thoughts?

And as to appropriateness - whether it's a hormonal level reaction to someone's appearance, or a visceral reaction to their scent, or an unconscious rumble of general hormonal energy for a teenager / young adult, or even just errant thoughts that are off topic in a more professional or military or legal setting... or daydreaming in a crucial lecture in a super-important class... I think that casual use of this faculy would be simply fraught with risks.

Moreover, you'd have to be a totally secure personality, with almost no significant quirks or hang-ups or neuroses to routinely allow others inside your thinking; and what if you were allowed into someone's mind for a discussion and in fact they were a raving paranoid with serious delusions - would you be able to differentiate enough to disengage? Would you be injured or scared or scarred?

There are telepathic "diseases" and/or "info-diseases".

They're rare, but there are dangerous thoughts that can be transmitted which just stick in your head. Someone who is suicidal or murderous or filled with hate can inspire that in others.

Emotions can spiral and be re-enforced by groups. Often that's a good thing
(sex and intimacy). However a fully linked in group can "flash mob" where they're all thinking the same irrational thing at the same time.

Ah, Telepathy. Being able to communicate with others at a distance without any external physical activity.

Why speak or communicate verbally/via gesture when you can share the concept itself that you want to share without any confusion?

Unfortunately, it's not that simple. Tobias mentioned a need to convert your message to a form that can be transferred, followed by decoding of the message. However, there is much more nuance and difficulty than just this.

First, Telepathy will be slow and exhausting, and will require extensive training to utilize effectively.

Let's say that you want to send the concept of a specific breed of cat to someone else. You, as the sender, must first conjure up the idea of what that breed of cat is. Memories will almost certainly not suffice for the details of what it looks like. Even trying to visualize the cat will take time and concentration.

Once the cat concept has been prepared, it must be sent. The recipient must be located, a link established, and the content packaged into a proper form and sent. The recipient must then decode the message - and there will probably need to be some kind of standard so that they know how exactly to do so. Drawing further inspiration from Network Protocol, there's also the issue of security and ensuring that data isn't compromised by some other person/third party. Though I doubt Man in the Middle attacks would be too much of a problem with Telepathy due to latency issues and the content of messages, it would still be necessary to encrypt and decrypt messages using some form of standard protocol, again adding more complexity.

Once this is done, an acknowledgement or response must be sent. Who knows just how much time it takes to have a single exchange?

In this timeframe, a few words (e.g. 'calico cat') when spoken would have sufficed.

Of course, the words 'calico cat' could have been sent via Telepathy. But this brings up a second issue - the expressiveness of Telepathy.

When humans communicate, facial expression, body language and gestures, intonation, etc. all convey meaning. Think of a phone call - how much information is lost when communicating by phone? And think of communicating by text on the internet? Everything but the words themselves is lost.

With Telepathy, emotions may be sent, and words and concepts may be sent, but sending all of the other information causes the message to bloat in complexity, going back to the first problem of Telepathy being slow, exhausting, and requiring training to use effectively.

It's just easier to communicate via speaking.

And as a final point, if someone is being contacted by multiple people at the same time, it will almost definitely result in significant mental stress. Will people have to block messages? Is it even possible to do so? Society would be a much more stressful place, especially if people who are blocked resort to shouting or yelling.

Why would people use spoken language over telepathy, given a native capacity for telepathy which can be polished and improved over time?

1. Different people organize their thoughts in different ways - and arguably in different internal "languages"

I am a sometimes visual thinker. That is, some of my reasoning is entirely without words, but the specific images, quasi-images, and system-connections I employ would be of no significance to anyone else. I have some reason to believe that other people may have more sophisticated emotional modelling than I do, and significantly worse systems-thinking.

It seems probable to me that if I were to drop certain of my thoughts directly into someone else's head, those thoughts would seem like gibberish. In order communicate, I have to first translate such an idea into English (often with a lot of added explanation of context). Telepathy would likely not bridge the natural context gap between individuals.

2. Telepathy may share more than was intended (or less, with misleading implications)

As alluded to in the first part of the answer, there may be a context gap when telepathically communicating sophisticated thoughts or feelings. That undertone of contempt for some relative or acquaintance? Are you also communicating the years of mistreatment you received... Or will you come off seeming unreasonable yourself?

Maybe you even are unreasonable, in ways you can hide from yourself, but which would be obvious to someone getting a taste of the thought without all of your own rationalizations.

3. Potential backflow of ideas might build up a social stigma around careless "wide-open" connections

If your telepathy is not perfectly one-way, it may be considered bad manners to open up an unguarded connection, in the same way touching (or putting your hands under the clothes of!) someone else is taboo EXCEPT in very specific situations. People are very careful, across most cultures, to not be too intimate; it's not your business to help yourself to other people's feelings or thoughts. This would be particularly the case if you can feel other people "snooping" in your head.

Telepathy would naturally be a very intimate connection, with the implication that it would be subject to the same taboos we have attached to other forms of intimacy.

Also,

4. Many thoughts are clearer if you can articulate them to yourself in words - and thus many people would be more able to communicate their thoughts better in words than in impressions anyway

To noisy or requires focus

"People can transmit clear thoughts, ideas, emotions and words through these communications."

The hard part of this question is that people can "transmit CLEAR ...". If I can assume the information relayed is clear and not confusing (I can send each word I would normally speak) I see multiple options but 2 that stand out. 1)

Just because I can speak clearly and relay information with no misunderstanding doesn't mean I can do this in any situation. What happens if no one is able to drown out thoughts or stop broadcasting them. You either are open to everyone's thoughts or you temporarily stop listening to everyone (like closing your eyes. You either see everything or nothing). The brain hasn't developed an ability to focus in on one person trying to communicate. In society where you are surrounded by people, this would cause you to rely on visual and verbal communication where your brain can help you isolate and focus a source (think of talking in a loud bar).

The second is strain. I might be able to do it but it requires great mental focus to use. Think of playing an instrument and dancing. Not to say it is impossible (clearly not) it just requires more training and focus than most people are willing to put into it. Without focusing on it people tuning (listening) into you would just get white noise. It has meaning but the brain is not complex enough (has yet to evolve) to decipher/understand it. With this I could have broken conversations with people over telepathy but without cause I would simply choose to communicate as we do currently.

Maybe because it's new technology. When texting came along, old people would still make calls for a long time after young people had caught on and had whole relationships over text messages. In your world telepathy might be a very accessible technology that anyone can use at any time but if most of your characters are over the age of 25 they will probably use other communication methods that they're already familiar with. They will only resort to telepathy when it suits the situation much more than anything else.