Correlation and Causation: Marriage, Poverty, and Teeth

January 15, 2014
Posted by Jay Livingston

Is dentistry destiny?

A couple of days ago, I commented (here) on the idea that marriage was the solution to poverty.  Or as the Heritage Foundation (and Sen. Rubio) put it,

Being raised in a married family reduced  child’s probability of living in poverty by about 82 percent.

The evidence for this assertion was a comparison of two-parent and single-parent families. Sure enough, a higher percent of single-parent families were poor. That was enough for PolitiFact to give Rubio a “Mostly True.” And now The Wall Street Journal has given Bush’s press secretary Ari Fleischer space to repeat this idea (“How to Fight Income Inequality: Get Married” - here).

Rubio, Heritage, Fleischer, and PolitiFact are committing a fallacy most undergraduates would see through in two seconds – mistaking correlation for causation. 

Here’s an analogy that comes via the Christian Science Monitor (here – it first appeared in October, but I discovered only yesterday):

“What is your social class? Take our quiz to find out!”

The quiz has thirty items, including this one

16. Have you visited a dentist in the past year?

A “Yes” answer counted towards the upper end of the social class scale. 

people with more than a high school education were twice as likely to have visited the dentist in the past year. Those living below the poverty line or without a high school education were also twice as likely to be edentulous, or toothless.

Or as Heritage-Rubio would put it, “Visiting the dentist once a year reduced a person’s probability of being poor by 50%.” 

Using the same conservative perspective, we can easily see the logic of the dentistry-poverty connection and its implication for policy.  People with bad teeth or no teeth wind up with bad jobs or no jobs. They are not attractive as potential employees. Because of their poor personal decisions regarding dental care, they suffer economically. If only they would visit the dentist annually, they would almost certainly rise from poverty. Needless to say, the government should not do anything directly to alleviate their poverty or dental care.  These are matters of personal virtue, and the government’s role should be only to exhort them to visit the dentist regularly.

That almost sounds reasonable. But the reverse causation is so much more likely. It’s not that having bad teeth causes poverty. It’s that taking yourself and your kids to the dentist regularly costs money – something poor people don’t have a lot of.

The dentistry-marriage analogy isn’t perfect, but it does illustrate the fallacy of assuming causation. It also points to something in the real world. The correlation between single parenthood and poverty is not automatic. It depends on government policies. In countries that provide low-cost childcare, medical care, and other benefits and services, single parents and their children will not suffer economically as they might under more punitive policies. Many countries have seen large increases in unwed parenthood – much greater than in the US – and their rates of single parenthood are greater.  (The graph is part of this CDC report.)

(Click on the chart for a larger view.)

But how are children in these countries faring?  The comparison of poverty rates shows a negative correlation – the higher the rate of unwed parenthood, the lower the rate of child poverty.*

(Child poverty data are from this OECD report.)

The question is not whether marriage and regular dental check-ups and other matters of personal virtue enhance economic prospects for parents and their children.**  The question is what governments should do for children who made the mistake of choosing parents who were not financially well-off, educated, healthy, and virtuous. 
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* The OECD measure of poverty is relative rather than absolute – the percentage of children in families with incomes less than 50% of the national median.  The measure could more accurately be termed a measure of inequality. However, in surveys in the US, when people are asked what they think the poverty line should be – i.e., what’s the minimum amount a family around here needs to just get by – the answer is usually a number that is about 50-55% of the median income.  So the OECD number does reflect widely-held ideas about poverty.

** The Oklahoma Marriage Initiative consisted of “federally-funded relationship skills training programs to promote marriage among unmarried parents.”   Philip Cohen at the Family Inequality blog outsourced the analysis of the Intiative to Kristi Williams, who writes (here) of these programs: “The conclusion: They have failed spectacularly.”