In his recent excellent blog post, Yvain discusses a few "universal" (commonplace) human experiences that many people never notice they don't have, such as the ability to smell, see some colors, see mental pictures, and feel emotions.  I was reminded of a longstanding argument I had with a friend.  She always insisted that she would rather be blind than deaf.  I could not understand how that was possible, since the visual world is so much richer and more interesting.  We later found out that I can see an order of magnitude more colors than she can, but have subpar ability to distinguish tones.  And I thought she was just being a contrarian for its own sake.  I thought the experience of that many colors was universal, and had rarely seen evidence that challenged that belief.  

More seriously, a good friend of mine did not realize he suffered from a serious genetic disorder that caused him extreme body pain and terrible headaches whenever he became tired or dehydrated for the first three decades of his life.  He thought everyone felt that way, but considered it whiny to talk about it.  He almost never mentioned it, and never realized what it was, until <bragging> I noticed how tense his expressions became when he got tired, asked him about it, then put it together with some other unusual physical experiences I knew he had </bragging>

This got me thinking about when it is likely we might be having unusual sensory experiences and not realize for long periods of time.  I am calling these "secretly secret experiences."  Here are the factors that might increase the likelihood of having a secretly secret experience. 

1) When they are rarely consciously mentally examined: experiences such as the ability to distinguish subtle differences in shades of color are tested occasionally (when choosing paint or ripe fruit), but few people besides interior decorators think about how good their shade-distinguishing skills are.  Others include that feeling of being in different moods or mental states, breathing, sensing commonly-sensed things (the look of roads or the sound of voices, etc.)  Most of the examples from the blog post fall under this category.  People might not notice that they over- or under-experience or differently experience such feelings, relative to others.  

2) When they are rarely discussed in everyday life: If my experience of pooping feels very different from other peoples' I may never know, because I don't discuss the experience in detail with anyone.  If people talked about their experiences, I would probably notice if mine didn't match up, but that's unlikely to happen.  The same might apply for other experiences that are taboo to discuss, such as masturbation, sex (in some cultures), anything considered gross or unhygienic, or socially awkward experiences (in some cultures).

3) When there is social pressure to experience something a certain way: it may be socially dangerous to admit you don't find members of the opposite sex attractive, or you didn't enjoy The Godfather or whatever.  Depending on your sensitivity to social pressure (see 4) and the strength of the pressure, this could lead to unawareness about true rare preferences.  

4) Sensitivity to external influences:  Some people pick up on social cues more easily than others.  Some notice social norms more readily, and some seem more or less willing to violate some norms (partly because of how well they perceive them, plus some other factors). I can imagine that a deeply autistic person might be influenced far less by mainstream descriptions of different experiences.  Exceptionally socially attuned people might (perhaps) take social influences to heart and be less able to distinguish their own from those they know about.  

5) When skills are redundant or you have good substitutes:  For example, if we live in a world with only fish and mammals, and all mammals are brown and warm and all fish are cold and silver, you might never notice that you can't feel temperature because you are still a perfectly good mammal and fish distinguisher.  In the real world, it's harder to find clear examples, but I can think of substitutes for color-sightedness such as shade and textural cues that increase the likelihood of a color-blind person not realizing zir blindness.  Similarly, empathy and social adeptness may increase someone's ability both to mask that ze is having a different experience than others, and the likelihood that ze will believe all others are good at hiding a different experience than the one they portray openly.

What else can people think of?

Special thanks to JT for his feedback and for letting me share his story.

Have you ever taken a personality quiz/test that helped you have valuable insights?  If so, what were the tests and how were they useful?

The only useful ones I've found all yielded the same type of insight.  They showed me where I stand relative to others, which is can be genuinely useful since representative samples of large populations can be hard to come by.  This includes IQ tests and tests for mental disorders (in my experience, people are usually aware that they are, for example, smarter than the average (although the Dunning-Kruger effect might complicate this) or have some intrusive thoughts and compulsive rituals, but might be surprised to find that they are three standard deviations above the norm or that their symptoms are sufficiently severe to be considered OCD). 

No remotely reliable (as in, not astrology) test I have ever seen has revealed genuinely surprising information for a moderately self-aware person, outside of ranking. Furthermore, they rarely gather personality data in a remotely subtle or non-transparent way ("do you like spending lots of time with large groups of people?" "yes..." "surprise, you're an extrovert!"), and thus seem super susceptible to test-takers' attempts to confirm a desired identity.  

An example of a more interesting/subtle way to potentially conduct a personality test would be to use question like OKTrends' "do you like beer?" which clusters strongly with "do you have sex on the first date," and, potentially, sexual openness. Such results might be harder for manipulate (consciously or unconsciously) and could assist with deeper self-awareness.   

Edited because the first link was broken.

What should a not-very-smart person do?  Suppose you know a not-very-smart person (around or below average intelligence).  S/he read about rationality, has utilitarian inclinations, and wants to make the world better.  However, s/he isn't smart enough to discover new knowledge in most fields, or contribute very much to a conversation of more knowledgeable experts on a given topic.  Let's assume s/he has no exceptional talents in any area.

How do you think a person like that could best use his/her time and energy?  What would you tell the person to do?  This person may be, compared to average LW readership, less capable of noticing the irrationality in his/her actions even if s/he wants to be rid of it, and less easily able to notice the flaws in a bad argument.  S/he may never be able to deeply understand why certain arguments are correct, certain scientific facts have to be the way they are, and telling him/her to be unsure or sure about anything seems dangerous if s/he doesn't really understand why.  

My practical advice might be:

1) If you want to give to charity, follow GiveWell recommendations.  

2) Learn about the basic biases, and commit to resisting them in your own life. 

3)  Follow advice that has been tested, that correctly predicts a positive outcome.  If a hypothesis is untestable (there's an unsensible dragon in your garage) or doesn't predict anything (fire comes from phlogiston in combustable substances), or is tested and demonstrably false (god will smite you if you say it doesn't exist), don't waste time and energy on it.  If you want to improve, look for tested methods that have significant positive results relevant to the area of interest.  Similarly, if a person regularly gives you advice that does not lead to good outcomes, stop following it, and if someone gives advice that leads to good outcomes, start paying attention even if you like that person less.  

At a more general level, my thoughts are tentative, but might include basic LW tenets such as:

1) Don't be afraid of the truth, because you're already enduring it.

2) If all the experts in a field agree on something, they might be wrong, but you are extremely unlikely to be better at uncovering the truth, so follow their advice, which might appear to conflict with...

3) Don't trust deep wisdom.  Use Occam's razor, think about simple, basic reasons something might be true (this seems good for religion and moral issues, bad for scientific ideas and understanding)

4) If you find yourself flinching away from an idea, notice that, and give it extra attention.  

Note:  I mean this as a serious, and hopefully non-insulting question.  Most people are intellectually near-average or below-average, and I have not seen extensive discussion on how to help them lead happier lives that make the world a better place.