Be careful when tweaking only one variable

Nice example of how tweaking only one variable can have an unintended effect on another:

“I began thinking about all this after reading an NBER working paper by Christopher Carpenter and Mark Stehr titled Intended and Unintended Effects of Youth Bicycle Helmet Laws. More than 20 states in the US have adopted laws requiring young people to wear a helmet when riding a bicycle. The authors found that these laws increased helmet use and reduced youth fatalities from cycling accidents by roughly 19 per cent, which is a good thing. But they also found that the laws significantly reduced, by between 4 and 5 per cent, the amount of cycling done by young people, which was not the intention.

“… The moral of this tale is to be careful when making policy, as you may get a response you didn’t plan for.”