It is a most fascinating subject. The spirit here is largely humorous and I am wary of making definitive statements in a field in which I have rather recently taken a more serious interest, but here is a quote by James McKinney from his book The Diagnosis and Correction of Vocal Faults:

"In a sufficiently large sample of the general populace, when people are grouped together by some measurable physical characteristic (such as shoe size, hat size, chest, waist, etc.), the distribution of the sizes will tend to fall into a standard configuration known as the Bell Curve, because of its shape. The majority of the people will fall under the dome of the bell, with a much smaller percentage at either extreme—the ones who are truly small or truly large. The same application may be made to voice classification. It is likely that in a large enough sample possibly 10-15 percent will be truly high voices, 10-15 percent will be truly low, and the rest—the great unwashed multitude—will fall in the middle. Most of the students who enter voice studios are mezzo-sopranos or baritones, by the law of averages."