What exactly does a 10-point difference in tested-IQ mean?

It means that, within a group of (supposedly) controlled tests, the scores of individuals or an individual varied by that many points. Practically speaking, it could mean that on one day someone had a cold, and on another day they didn't.

IQ is essentially meaningless except as a statistic. What you do with what you have is far more important than numbers generated by a test that, in most ways, has nothing to do with the real world.

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Updated 08/22/2013 - On a realistic and human level it would depend greatly on what your perspective may be. For example, a 10-point difference could be quite significant from a relationship perspective. One partner may feel vastly superior in intelligence or as if they are victim to some hidden agenda that's led them down a dark road of deceit ultimately tricking them into marrying a 3rd world, low-class, dimwit. This might translate into the more intelligent partner honestly believing they are in a relationship with someone who is unable to navigate the relationship to healthier choices and optimal conditions of what a true and almost perfect relationship should be. This, of course, could be catastrophic for the relationship. What I have learned is that IQ points do not matter when you pass a particular point. From the 'low to high' everyone possess some form of genius. And it's a matter of discovering that in each person that will reveal to you what his or her real strength may be. But remember, it is in our flaws, imperfections and weaknesses that we find and bond with each other.

But perhaps a more intelligent response to your exact question may more closely resemble an answer of 'A 10-point difference in tested-IQ means nothing because your question has been improperly formatted.' The answer to the question must be constrained due to dependencies in the accurate interpretation of your question, knowledge and intent.