Wednesday, May 16, 2012

Correlation and Causation: the Teen Pregnancy Edition

One of the first posts I ever did was on correlation and causation.  In it, I spelled out the three rules to consider whenever two variables (x and y) are linked:

  1. X is causing Y
  2. Y is causing X
  3. Something else is causing both X and Y
While most people jump to the conclusion that it's number 1, Matthew Yglesias wrote a piece for this week where he rather awkwardly jumps to conclusion number 2.  

He starts off well with the second paragraph, but then goes to very strange place in the third: 

Delivering the commencement address last weekend at the evangelical Liberty University, Mitt Romney naturally stuck primarily to “family values” and religious themes. He did, however, make one economic observation that intersects with some fascinating new research. “For those who graduate from high school, get a full-time job, and marry before they have their first child,” he said, “the probability that they will be poor is 2 percent. But if [all] those things are absent, 76 percent will be poor.”
These are striking numbers, but they raise the age-old question of correlation and causation. Does this mean that the representative high-school dropout would be doing much better had he stuck it out in school for a few more years? Or is it instead the case that the population of high-school dropouts is disproportionately composed of people who have attributes that lead to low earnings?
When it comes to early pregnancy, surprising new evidence indicates that Romney and most everyone else have it backward: Having a baby early does not hamper a young woman’s economic prospects, as Romney implies. Rather, young women choose to become mothers because their economic outlook is so objectively bleak.
Say what?

As a former teenage girl myself, this is a strange conclusion....I certainly never met a teen mom who would have put it that way.  But surely there was some wonderful evidence to support this scathing conclusion?

Well, not really.  Here's the original paper....and  here's how the authors conveyed their thoughts:

We describe some recent analysis indicating that the combination of being poor and living in a more unequal (and less mobile) location, like the United States, leads young women to choose early, non-marital childbearing at elevated rates, potentially because of their lower expectations of future economic success. ...These findings lead us to conclude that the high rate of teen childbearing in the United States matters mostly because it is a marker of larger, underlying social problems.
The emphasis was mine....but notice how much more careful they are in their language.  If you take my list above, you see that they are challenging possibility number 1, seeing if #2 is a feasible conclusion, but ultimately pointing the finger at #3....i.e. "larger, underlying social problems".

For example, the cite low maternal education as a risk factor for teen pregnancy...which one could presume could be either the result of or the cause of low income.

Teen pregnancy is complicated, and honestly I would be very surprised if you could ever figure out a way to pin it on just one factor.  Additionally, so much information is unavailable that it can be hard to parse through what you have left.  A key factor in all of this would be to determine if higher income girls weren't having babies because they weren't getting pregnant or because they were having which could lead to very different conclusions.

I fully support this study, by the way, questioning the prevailing wisdom is always a good thing. What I resent is when people think just by flipping the order of a normal conclusion that they're being clever.

X could cause Y, Y could cause X, something else could be causing both.

Then again, it could also just be a coincidence.  


  1. I always think I'm being clever when I flip the conventional belief of causality.

    I am always suspicious the same set of "root causes" being trotted out all the time, however. Underlying social problems...I'll bet I can make a guess what that means.

    Not that those can't be true, or at least partial explainers of the trend. But the probable list of underlying social problems - poverty, bad schools, racism, lack of mobility - were all far worse sixty years ago. In a way, it is supporting Charles Murray's Losing Ground via the back door. If having a child out of wedlock is a girl's best option, we're doing something wrong.

    1. "I always think I'm being clever when I flip the conventional belief of causality."

      No. You just enjoy disagreeing with people.

      As for the underlying causes...I agree they can be tired and retread, but they also give latitude for real thought. Dan encountered some truly tragic teen pregnancy cases this year (like 14 year olds with 1 year old children tragic), and we had a good talk about the role of untreated mental illness in the whole thing. The girl was ragingly bipolar, but her drug addicted (30 year old) mother couldn't help, the grandmother (who was 45) couldn't pull it together enough to intervene, etc.

      It was noteworthy to me because every kid in his school is poor and almost all are minorities, and the ones with stable parents and few other risk factors still do better regardless of income. That's why the "poverty" panacea bothers me. There's a lot of different ways of being poor.

  2. I'm the creator of Correlated and just wanted to thank you for the shout-out.

    Oh, and also, I wanted to defend the soundness of my correlations. If they were really mere coincidences, as you suggest, don't you think I would have named the site Coincidenced?

    1. Thanks for stopping by Shaun!

      I did think your website was fun, and I would not question the soundness of your correlations. However, without the p values, I could question whether they were statistically significant or not....but I won't. Your website has informed me that this line of thinking means I suck the fun out of everything. I don't like sucking fun out of things, the world's too serious as is.

      In conclusion, I don't often question correlations, only causality....though I do think "Questionable Causality" might have been a pretty good name for a website too. Or a band.