John Allen Paulos in his Who's Counting column at ABC News:
Oddly enough, the shape of normal bell-shaped (and other) statistical curves sometimes has unexpected consequences for such situations. This is because even a small divergence between the averages of different population groups is accentuated at the extreme ends of these curves, and these extremes, as a result, often receive a lot of attention.
There are policy inferences, most of them wrong-headed, that have been drawn from this fact, but I certainly don't want to delve into questionable claims. I merely want to clarify a couple of mathematical points. To illustrate one such point, assume two population groups vary along some dimension – height, for example. Let's also assume that the two groups' heights vary in a normal or bell- shaped manner. Then even if the average height of one group is only slightly greater than the average height of the other, people from the taller group will make up a large majority of the very tall.
Likewise, people from the shorter group will make up a large majority of the very short. This is true even though the bulk of the people from both groups are of roughly average stature. So if group A has a mean height of 5'8″ and group B a mean height of 5'7″, then (depending on the variability of the heights) perhaps 90% or more of those over 6'3″ will be from group A. In general, any differences between two groups will always be greatly accentuated at the extremes.
More here.