Truth & social graces

The phrase "how are you" is no different from a TCP Handshake. In order to establish communication, the first initiating computer will send out a synchronize, the responder sends a synchronize-acknowledge, then the initiator will give back a single acknowledge. This is just how human language does it, with a slight order change.

"How are you?" - Synchronize request

"Good, how are you?" - Acknowledge synchronize request, send sync request of your own

"I'm good too" - Acknowledge

Just like in computers, the purpose is not to convey information in itself. It is to establish communication including the rules of communication each side will be using. If you want to change the transfer protocol over to UDP, that is also acceptable (eg, "how are you?" "ugh terrible! ). You can also throw up halt flags before conversation begins (eg, "how are you?" "sod off"). The initial synack is also good for pinging (eg, "how are you?" ... "sorry I was busy thinking").

However, the one thing that neither humans nor computers use synack for is transmitting information. Doing so is simply a breach of information transfer protocol and may result in you defecting by accident.

I've gotten in the habit of saying "I can't complain" when someone asks me how I am, since "I'm fine" is rarely the truth. I like that response because it's kind of meta the way I use it: in my head it's short for "I can't complain to you because you don't actually want to know and being made to listen to my complaints would alienate you."

I have a little kid who is generally quite truthful, innocent enough not to lie in most cases. I noticed recently that when someone asks him, "How are you", he usually answers in detail because, well, you asked, didn't you? When I was a teenager I hated people who lied and I tended to ignore these unwritten social rules to the extent I could. I.e. I didn't ask if I didn't want to know and people thought I was rude. So, my question is, should I teach him to lie upon these occasions?

It becomes clear that no lying is necessary in this case when you realize that the actual purpose of producing the sequence of sounds "how are you" is often not to request a detailed description of the listener's current well-being, but only to greet them, foster good-will, show friendship, or something like that.

You must simply know the context. Sometimes the word "box" points to an object; other times to a sport. Similarly, sometimes the phrase "how are you" functions as a request for a detailed etc; other times it's for something somewhat different.

The habit of asking "how are you" when one is not really interested in the answer is not a human universal; it's a distinctly Western phenomenon.

You can see this pointed out, for example, in Brat 2 (although the scene in question also goes on to engage in some rather less-accurate anti-American stereotyping).

It accords with my own experiences in Russia, which are that you should not ask this unless you are ready to accept an earful.

My rule of thumb is something like: if I have to worry about it being uncovered, it's a (morally wrong) lie. If I don't care if it's uncovered, it doesn't really count as a lie. If I'm sick and frustrated and angry and tired and someone asks me how I am and I answer "I'm OK", it may be an incorrect representation of reality, but not one that I will try to protect from further investigation.

That conveniently extends to Santa Claus - if my son investigates the Santa Claus story and says it's false, I won't come up with more details to cover it up (unless I can come up with something sufficiently ludicrous that I can say with a straight face).

Teach him how the custom works ("how are you" doesn't mean much more than "hello"), not just a rote response (when someone says "how are you", answer "how are you").

ETA: And when he asks why, don't say "because it's polite", which is like explaining that things fall down because of gravity.

Your definition of a lie is unusual. Lie is generally defined as intentional deception. If you do not tell people how you are when they do not want to know, you are not lying. Teach him the difference between form ("Hi, how are you?") and substance ("Greetings, fellow earthling, I am happy to not be alone on this planet"), and that replying simply with "Hi" is completely appropriate and is in no way lying.

As for actual lying (intentional deception), you should certainly discourage it in most cases, as there are nearly always better alternatives.

More broadly, I was thinking, why am I committed to being truthful, in general?

I would of course need to know more about you to be in a position to know the answer, but I can venture a guess by relating why I personally have the same tendency.

Long story short, other-deception seems to go hand in hand with self-deception. It's extraordinarily epistemically hazardous to lie. Because I value my epistemic rationality to such a great extent, I would use dishonesty only as an absolute last line of defense.

As has been pointed out by Xachariah and Crux, perhaps you should think of "How are you?" as a set of symbols with multiple potential meanings rather than one whose meaning is determined by its literal interpretation.

Consider (yet another) analogy: homonymy across languages. Imagine that the sound "yeridan" refers in Language A to a puppy and in Language B to an army of bullet ants. If a speaker of A asked you whether you wanted to hold a yeridan, you might answer that you would, while you surely would decline if the one doing the asking were a speaker of B.

Similarly, we have (far more than) two languages, or registers, at work here. One is the language of social grace, to be used when signaling certain aspects of our social situation (e.g., loyal to whom, intimate with whom, aloof toward whom). When conversing in this language, the function of "How are you?" is to initiate conversation and display basic good will. The other language is the language of intimacy, to be used when giving details about our lives to others. In it, "How are you?" functions as a query about recent goings-on and one's general situation.

The meaning of "I'm fine" as a response similarly differs depending on the context, just as the meaning of "Yes, I would like to hold a yeridan" differs depending on who's asking. Saying "I'm fine" in a social-grace context is just as truthful as saying "No yerdidans for me, thanks" in a Language B context, regardless of how one would respond in the context of intimacy or Language A.

I don't usually find that lying is necessary in situations like this.
For example, when I'm at my lowest, my usual response to "How are you?" is "Eh... been better. You?"
No doubt there are communities in which even that is too much actual information; I don't hang out in such communities by preference, but if I found myself in one I'd give an even less informative answer.
As for what to teach kids... well, beats me, but I will say that knowing when a question doesn't merit a complete and exhaustive answer is a social skill worth learning.

Why interfering and not letting your kid develop his own ways? Answering "How are you?" in detail sounds to me as a fantastic trait of his personality.

When I was 7-years old I stopped calling my parents mom and dad and switched over to calling them by their names. I just couldn't understand the logic of other people call them one thing and me calling the something else. Happily nobody tried to "correct" me according to social rules, and still today it wouldn't cross my mind to call my mother 'mother'!

Nobody ever actually told me, as a kid, that I should try modeling people, try getting inside their heads, as a mode of navigating social situations and understanding the world in general. Empathy develops gradually on its own in children, and we are generally left to just figure out that it's good for something other than making us feel guilty.

I suggest you do not say to your child, "Lie more." Instead say, "Try to imagine you are other people. Ask what states of mind they might be in that could make them say what they say and do what they do."

This could lead to the child understanding when lies actually matter in an instinctive way.

As a general rule, when I ask "how're you?" I actually mean the question, and I want an actual answer if the person I'm asking is willing to give it.

So yeah, I know it's used as a "social lubricant", but I myself generally ask it with the intent of actually finding out how the person is doing.

Maybe your child is an INTP

On the other hand, their ability to grasp complexity may also lead them to provide overly detailed explanations of simple ideas, and listeners may judge that the INTP makes things more difficult than they need to be. To the INTPs' mind, they are presenting all the relevant information or trying to crystallize the concept as clearly as possible.

I was definitely like this and I still am, although I'm finally getting better. I was always honest to a fault, especially as a child. In fact, I didn't even want to say things that were true but insincere, because I felt implicitly that it was too close to lying. So if I was eating a delicious meal, and everyone around the table was saying something nice, I would hesitate because I knew that I was expected to say something, but I may not have normally.