What is reverse causation?

Reverse causation (also called reverse causality) refers either to a direction of cause-and-effect contrary to a common presumption or to a two-way causal relationship in, as it were, a loop.

Reverse causation can occur when people change their diet or other lifestyle habit after developing a disease or perhaps after having a close family member suffer an event like a heart attack.

Here is a good example of reverse causation:

When lifelong smokers are told they have lung cancer or emphysema, many may then quit smoking. This change of behavior after the disease develops can make it seem as if ex-smokers are actually more likely to die of emphysema or lung cancer than current smokers.

It is also known that many alcoholics become nondrinkers after being diagnosed with cirrhosis of the liver. Such changes can confuse or confound what is a causal correlation between dying of a disease and an environmental factor.

Clearly, the decisions by smokers and alcoholics to stop smoking or drinking did not cause them to get sick and die. Even so, this tendency to reduce or eliminate things that make you become sick after the fact can undermine the statistical correlation between smoking and dying of lung cancer, and heavy alcohol consumption and dying of liver disease.

Here is another good example of reverse causation:

When people become concerned about their high risk of having a heart attack because of a family history of heart disease, or chest pain, or being told by their doctor they are at higher risk, they are more likely than the general population to reduce their intake of foods high in saturated fat and cholesterol. So, while these people’s current diets may be lower in saturated fat and cholesterol, they may still be more likely to have a heart attack.

Clearly, their cutting back on saturated fat and cholesterol-rich foods did not increase their risk of having a heart attack or heart surgery to open blocked coronary arteries. Nevertheless, statistically, these high-risk people may be more likely to have a heart attack or heart surgery despite their now healthier-than-average diets.

Observational studies (which are studies that ask people what they are eating today and then observe who has a heart attack or gets heart surgery over the next few years) cannot differentiate between subjects who have an increased risk of such cardiac events despite their recent change of diet. In this case, fear of disease caused them to improve their diet rather than their improved diet causing their cardiac event. So reverse causation occurs when a heightened risk of disease leads people to make diet and/or lifestyle changes in hopes of reducing that risk, but those changes were either too little or came too late. This is one reason observational studies cannot tell us what caused what.​