[galadriel]

How did you get the 15% number? Assuming IQ of 70 is taken as margin of borderline intellectual functioning, and 15 points being the standard deviation, only people below two standard deviation lie below 70 IQ, or about 2.5% of population. (The other 2.5% being above 130 IQ)

[Daniel_Newby]

70 is generally taken as the threshold for mild mental retardation. I was using 85 as the threshold for borderline and assuming a non-normal distribution.

[galadriel]

You have got your thresholds wrong. http://en.wikipedia.org/wiki/Mental_retardation#IQ_below_70

Also, the distribution is by definition normal, and I have hard time believing 15% of population is retarded. That would mean about 1 in every 6 person I met is retarded, a number too big to believe. Now 3 genius/3 dumb out of 100 is more like what my experience has been.