Paul H. Byerly
Most measurable characteristics of biological things fall on what's known as a bell curve. The X axis is the measure of the thing, with the smaller or least on the left and the larger or the most on the right. The Y axis represents the number of individuals of the given size or quantity. The majority of individuals are clustered towards the center, with fewer and fewer individuals towards each extreme. The curve can be wide and short or narrow and tall, but the center remains the point where the most individuals fall.
Below are two sets of bell curves over laid on each other. These sets of curves are typical of the gender differences found in many biological traits.
In the top curve, one gender has a broader curve, while the other has a higher curve. In humans, IQs fall on a set of curves like this, although not nearly as exaggerated as the example. In general there are more males on both ends of the IQ curve, and more women in the center.
In the bottom set of curves, one gender is stronger in the characteristic than the other. While some of the reds are stronger than some of the greens, the majority of the greens are stronger than the majority of the reds. In humans the sense of smell falls on a set of curves like this, although again not as exaggerated as shown. In general women have a more sensitive sense of smell then men.
Of course these are simplified examples. Usually the curves will be both different and offset. In addition the difference can be so small as to be relevant only from a statistical point, or very great.
Image Credit: © Paul H. Byerly