Gifted is just a label to a certain degree which is degraded in concept by bureaucratic school standards, normative scores, competitive wishful thinking, and ham-handed political correctness.
Setting aside the label... you have brain, psychological, and enviornmental factors that inherently have normal distributions. No particular characteristic is perhaps innately sufficient or necessary (beyond some minimum thresholds) for extreme intellectual capabilities, but when multiples occur together you tend to get intellectual performance which is labeled gifted.
Because of these factors working together, the more common ones are then rolled into gifted characteristics checklists along with secondary chacteristics which result naturally from those bumping up against life.
Some of the factors: cortical minicolumn density and distribution, dopamine responsiveness, sensory developmental disruptions. The normal distribution of minicolumns maps inversely into intellectual giftedness such that the two tail distributions are both seen commonly in giftedness. These two extremes also correlate with autism on one end and dyslexia on the other. Dopamine responsiveness is one possible causal area for overexctiabilities, where the world, senses, and emotions are experiencd more intensely.
I'll second reading The Dylexic Advantage.
