MG. Good question; where indeed is the evidence? IQ scores are often glibly stated (= assumed) to follow a Gaussian distribution, but it's very hard to track down raw data or to discover the characteristics of the study population. It's a mess and is probably too PC-sensitive anyway. I'd put good money on the distribution having a fatter tail of low scores – the result of developmental defects, injury, illness, etc. Quite a lot of brains get damaged by unfortunate events, but few of such events increase IQ. It's unfortunate that many (most?) medical data is assumed to follow a Gaussian distribution when it doesn't. This is assumption is really for the convenience of stats calculations, and, of course, is 'good enough' because it covers the majority of the population.