5. As the text explains, it can sometimes be very difficult to sort out the direction of causality.

a. Why might you think that more police officers would lead to lower crime rates? Why might you think that higher crime rates would lead to more police officers?

b. In 2012, the New England Journal of Medicine published research that showed a strong correlation between the consumption of chocolate in a country and the number of Nobel Prize winners in that country. Do you think countries that want to encourage their citizens to win Nobel Prizes should increase their consumption of chocolate?

c. A recent article in the Journal of Applied Physiology found that elderly runners had healthier muscles than a comparison group of the same age. Although the members of the comparison group were all still living independently, they had lower muscle mass and muscle strength than the athletes. The popular press framed the article as proof that exercise causes people to be healthier. Is that the only way to interpret causality in this example?