My last paragraph above ought to read    "Physical traits won't find all XXY boys, nor will educational difficulty.  The absolute best way to find all XXY boys is a Barr Body test at birth.  You can still maintain your myth that XXY boys have Klinefelters' syndrome, no loss of pride there to admit you're wrong, and the boys get the assistance they need if they show they need assistance.  Then you can develop a truer model for finding  XXY boys in childhood, for countries that can't afford Barr Body tests."

For those who don't know, Barr bodies are the shadow of the inactivated X. So if you have a  child with male genitals and a Barr Body, you know you have a boy with XXY sex chromosomes, and if you have a child with female genitals and 2 Barr Bodies you know you have a girl with 3 X chromosomes.  Once the children have been identified with Barr Bodies, or extra Barr Bodies, then further more expensive, genetic testing can take place.  

The idea that additional X's in males will lead to absolute differences physically in childhood is wrong.   If your researchers have ever been to an conference of XXY males, adults and children, and their parents you will see absolute proof such a system of discovery based on physical differences will not find all XXY's.  So why don't researchers get out of the laboratory  and into the real world, and meet the people?