False Cause: Ignoring Common Cause

False Cause: Ignoring Common Cause

Bertha Alvarez Manninen

There is increasing evidence that the easy availability of welfare has greatly increased the incidence of child poverty. For example, the highest increases in the rate of child poverty in recent years have occurred in those states that pay the highest welfare benefit.

White House Task Force Report, 2013

In general, the false cause fallacy occurs when the “link between premises and conclusion depends on some imagined causal connection that probably does not exist” (Hurley 2015, 149). There are three different ways an argument can commit the false cause fallacy: post hoc ergo propter hoc; cum hoc ergo propter hoc; and ignoring common cause. Ignoring common cause is the focus here. Related discussions cover post hoc ergo propter hoc (Chapter 80) and cum hoc ergo propter hoc (Chapter 78).

The fallacy of ignoring common cause occurs when one notices a constant correlation between A and B and assumes A caused B (or vice versa) while ignoring that there is a third variable, C, that causes both and therefore accounts for the correlation. As Sheila Morton (2014) notes, the quotation above from the White House Task Force report commits this version of the false cause fallacy because “the report ignored many other, more probable, causes of the rising rates of childhood poverty: inflation, job loss, the mass‐movement of manufacturing to overseas plants that took place during the Reagan era. All these are more likely causes of childhood poverty than the availability of welfare” (98). Indeed, these reasons can explain both the rise of childhood poverty and the rise of the availability of welfare without making the latter causally responsible for the former.

For a smoking example related to Chapter 78, consider the following: “coffee drinking and lung cancer are associated, but coffee drinking doesn’t cause lung cancer, nor vice versa. They are associated because both are associated with cigarette smoking. Smokers happen to drink more coffee on average than non‐smokers” (McKay Illari et al., 2011, 16). In this case, the tendency to smoke is the common cause that accounts for the correlation between excessive coffee drinking and lung cancer. Ignoring this common cause in an observational study (as was done in the example in Chapter 78 concerning HRT and coronary disease) could lead to the conclusion that it is the coffee drinking itself that increases the risk of lung cancer (thus instilling an unfounded fear among coffee drinkers who are non‐smokers).

The phrase “breast is best” expresses a broad claim about infant nutrition. A narrower causal question concerns the positive correlation between breastfeeding and children’s IQ scores: does the correlation establish that breastmilk itself increases intelligence? One concern is that asserting this causal relationship ignores a possible confounder (a third variable that influences both variables being studied) – that women who breastfeed “also often provide a more enriched and cognitively stimulating environment for the child” and that “breastfeeding is associated with higher socio‐economic factors that may be the true cause of enhanced intellectual ability” (Jacobson and Jacobson 2002, 258). In other words, it may not be the breastmilk itself that contributes to a child’s higher IQ scores; rather, the tendency toward breastfeeding and providing a mentally stimulating environment for a child has a common cause: membership in a higher socio‐economic group.

An association between marijuana use and later use of drugs such as heroin or cocaine also raises a common-cause question. Social circumstances, psychological factors, or access to drugs might influence both. The sequence of use alone does not establish that marijuana caused the later use, nor does identifying possible common causes establish that they explain the entire association.

Sometimes it isn’t clear exactly which version of the false cause fallacy applies. An advertisement for Eukanuba dog food implies that Utah, a 17‐year‐old dog, has lived for 30% longer than his expected lifespan because he has been fed this brand. There is no indication that any kind of substantial study has been done to illustrate that Eukanuba dog food causally contributes to a dog’s increased lifespan, and so, as it stands, the ad commits the false cause fallacy. If what the advertisers mean to say is that the causal connection exists because eating the food precedes the dog’s increased lifespan, then this is a post hoc ergo propter hoc fallacy. If what they mean to say is that the two (the dog’s diet and his increased lifespan) occur simultaneously and therefore a causal connection can be inferred from this alone, then the ad commits the cum hoc ergo propter hoc fallacy. In addition, the ad may be ignoring that there could be a common cause that explains both the dog’s diet and his longer lifespan – perhaps the dog has the kind of owners who are particularly mindful about their dog’s health and this contributes both to buying expensive food for him and also to ensuring his health in other ways. In any case, there is no reason to believe from this ad alone that this particular brand of dog food itself has a direct causal connection to Utah’s increased lifespan.

In 2009, the Kellogg Company settled against charges that its advertisements for Frosted Mini‐Wheats were deceptive. The ad claimed that the cereal was “clinically shown to improve kids’ attentiveness by nearly 20%” (Federal Trade Commission 2009). What it neglected to mention is that the study compared children who had eaten the cereal to children who hadn’t eaten any breakfast at all. That comparison does not isolate a benefit specific to Frosted Mini‐Wheats. A comparison with other breakfasts would be needed to assess whether the cereal offers an advantage over them. The commercial exploited the false cause fallacy (in particular the post hoc ergo propter hoc) to get consumers to buy its product. This fallacy is also present in many of the arguments concerning the alleged connection between vaccines and autism despite the plethora of evidence otherwise. Causal mistakes can affect purchasing, health, and policy decisions.

For further reading, Tyler Vigen’s (2015) website Spurious Correlations is a humorous illustration showing that correlative events are not necessarily causal ones, as he presents various graphs of two events that have a correlative, but clearly non‐causal, relationship. For example the “number of people who drowned by falling into a swimming‐pool correlates with [the] number of films Nicolas Cage appeared in [… the] per capital cheese consumption correlates with [the] number of people who died by becoming tangled in their bedsheets.” Martin Myers and Diego Pineda’s (2008) book Do Vaccines Cause That?! offers excellent data concerning the efficacy of vaccines and highlights how the false cause fallacy is committed by anti‐vaccination advocates when trying to connect autism with vaccinations. The website logicalfallacies.info is an excellent compilation of various informal and formal fallacies, including the different variations of the false cause fallacy.