Crime and Abortion :: Correlation or Causality?

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Crime and Abortion :: Correlation or Causality?

n a recent radio commentary (John Robbie, on 702 on the 20th of September 2007) it was noted that research is coming out of the United States and also the UK that suggests that the legalization of abortion could have something to do with the decrease in certain types of crime. One of the most popular treatises on this topic is by economist Steven D. Levitt and journalist Stephen J. Dubner, calledFreakonomics: A Rogue Economist Explores the Hidden Side of Everything.

At first glance one could draw a conclusion that since there there would be fewer “unwanted” children, and since the likilihood of these children committing certain types of crimes is high, the incidence would decrease.

From a qualitative perspective one could make these crude deductions. However, we must be careful. Fortunately, Levitt and Dubner don’t just take a “first glance”.

Statistically what is done to test such hypotheses is to see whether there is acorrelation between the two factors. So what if a correlation is found? All we then know is that the two issues are related in some way. Do we know what causes what? To determine the nature of the cauasality we need more data. For starters, we need to determine if one of the correlated factors is independent.

But most importantly, we need to rule out the fact that both factors are not in fact caused by some underlying third factor. Indeed when we start to examine the possibility of additional variables, they become so numerous that normal deterministic methods become powerless.

Some reports are brave enough to assert, however: “The fragility of the results in this paper serve to emphasize the difficulty researchers have in identifying causal effects of social change such as abortion legalization on crime rates some years hence, particularly given the myriad of other social changes occurring over the same time and which may dilute any effect.”

Let us not persist the common error of confusing correlation with causality.

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