The Bell Curve has become such a fixture in both research and everyday situations that it is often over-used and mis-used. I will wait for another day to talk about that topic specifically; here, I want to suggest that bell curves rarely be shown in a chart, and never more than one bell curves on one chart.
I thank the Truck and Barter blog for bringing my attention to the paired bell curves in the World Bank Malnultrition Report. Professor Gelman's comments started me thinking about this.
There is serious distortion in this presentation. If you recall your first stats class, it is the tail probabilities, not the height of the curves, that matter. Unfortunately, curves like these tend to draw our attention to the heights.
You might also recall that the mean and the dispersion together completely define any normal distribution curve so really, the only salient features of each curve is the "center" of the curve (in this case, 0 vs -1.8) and the "width" of the curve (here, 6 units vs 8 units). Sadly, while the labels are numerous, they do not point out these salient features.
I wouldn't be so insistent were it not for the fact that Tukey had long ago invented a far superior way to show distributions. Here is a boxplot style representation of the same information:
The chart is not quite to scale, and the vertical axis dimension is missing. Plus the length of the box is not the usual interquartile range. But the center and width of each curve are clearly shown, and their relative sizes easily read off the chart.