Causal notation

In nature and human societies, many phenomena have causal relationships where one phenomenon A (a cause) impacts another phenomenon B (an effect). Establishing causal relationships is the aim of many scientific studies across fields ranging from biology and physics to social sciences and economics. It is also a subject of accident analysis, and can be considered a prerequisite for effective policy making.

To describe causal relationships between phenomena, non-quantitative visual notations are common, such as arrows, e.g. in the nitrogen cycle or many chemistry and mathematics textbooks. Mathematical conventions are also used, such as plotting an independent variable on a horizontal axis and a dependent variable on a vertical axis, or the notation ${\displaystyle y=f(x)}$ to denote that a quantity "${\displaystyle y}$" is a dependent variable which is a function of an independent variable "${\displaystyle x}$". Causal relationships are also described using quantitative mathematical expressions, which can be linear or nonlinear, and can be visualized (See Notations section.)

The following examples illustrate various types of causal relationships. These are followed by different notations used to represent causal relationships.

Examples

What follows does not necessarily assume the convention whereby ${\displaystyle y}$ denotes an independent variable, and ${\displaystyle f(y)}$ denotes a function of the independent variable ${\displaystyle y}$. Instead, ${\displaystyle y}$ and ${\displaystyle f(y)}$ denote two quantities with an a priori unknown causal relationship, which can be related by a mathematical expression.

Ecosystem example: correlation without causation

Imagine the number of days of weather below one degrees Celsius, ${\displaystyle y}$, causes ice to form on a lake, ${\displaystyle f(y)}$, and it causes bears to go into hibernation ${\displaystyle g(y)}$. Even though ${\displaystyle g(y)}$ does not cause ${\displaystyle f(y)}$ and vice versa, one can write an equation relating ${\displaystyle g(y)}$ and ${\displaystyle f(y)}$. This equation may be used to successfully calculate the number of hibernating bears ${\displaystyle g(y)}$, given the surface area of the lake covered by ice. However, melting the ice in a region of the lake by pouring salt onto it, will not cause bears to come out of hibernation. Nor will waking the bears by physically disturbing them cause the ice to melt. In this case the two quantities ${\displaystyle f(y)}$ and ${\displaystyle g(y)}$ are both caused by a confounding variable ${\displaystyle y}$ (the outdoor temperature), but not by each other. ${\displaystyle f(y)}$ and ${\displaystyle g(y)}$ are related by correlation without causation.

Physics example: a unidirectional causal relationship

Suppose an ideal solar-powered system is built such that if it is sunny and the sun provides an intensity ${\displaystyle I}$ of ${\displaystyle 100}$ watts incident on a ${\displaystyle 1}$m${\displaystyle {}^2}$ solar panel for ${\displaystyle 10}$ seconds, an electric motor raises a ${\displaystyle 2}$kg stone by ${\displaystyle 50}$ meters, ${\displaystyle h(I)}$. More generally, we assume the system is described by the following expression:

${\displaystyle I\times A\times t=m\times g\times h}$,

where ${\displaystyle I}$ represents intensity of sunlight (J${\displaystyle \cdot }$s${\displaystyle {}^{-1}}$${\displaystyle \cdot }$m${\displaystyle {}^{-2}}$), ${\displaystyle A}$ is the surface area of the solar panel (m${\displaystyle {}^2}$), ${\displaystyle t}$ represents time (s), ${\displaystyle m}$ represents mass (kg), ${\displaystyle g}$ represents the acceleration due to Earth's gravity (${\displaystyle 9.8}$ m${\displaystyle \cdot }$s${\displaystyle {}^{-2}}$), and ${\displaystyle h}$ represents the height the rock is lifted (m).

In this example, the fact that it is sunny and there is a light intensity ${\displaystyle I}$, causes the stone to rise ${\displaystyle h(I)}$, not the other way around; lifting the stone (increasing ${\displaystyle h(I)}$) will not result in turning on the sun to illuminate the solar panel (an increase in ${\displaystyle I}$). The causal relationship between ${\displaystyle I}$ and ${\displaystyle h(I)}$ is unidirectional.

Medicine example: two causes for a single outcome

Smoking, ${\displaystyle f(y)}$, and exposure to asbestos, ${\displaystyle g(y)}$, are both known causes of cancer, ${\displaystyle y}$. One can write an equation ${\displaystyle f(y)=g(y)}$ to describe an equivalent carcinogenicity between how many cigarettes a person smokes, ${\displaystyle f(y)}$, and how many grams of asbestos a person inhales, ${\displaystyle g(y)}$. Here, neither ${\displaystyle f(y)}$ causes ${\displaystyle g(y)}$ nor ${\displaystyle g(y)}$ causes ${\displaystyle f(y)}$, but they both have a common outcome.

Bartering example: a bidirectional causal relationship

Consider a barter-based economy where the number of cows ${\displaystyle C}$ one owns has value measured in a standard currency of chickens, ${\displaystyle y}$. Additionally, the number of barrels of oil ${\displaystyle B}$ one owns has value which can be measured in chickens, ${\displaystyle y}$. If a marketplace exists where cows can be traded for chickens which can in turn be traded for barrels of oil, one can write an equation ${\displaystyle C(y)=B(y)}$ to describe the value relationship between cows ${\displaystyle C}$ and barrels of oil ${\displaystyle B}$. Suppose an individual in this economy always keeps half of their value in the form of cows and the other half in the form of barrels of oil. Then, increasing their number of cows ${\displaystyle C(y)}$ by offering them 4 cows, will eventually lead to an increase in their number of barrels of oil ${\displaystyle B(y)}$, or vice versa. In this case, the mathematical equality ${\displaystyle C(y)=B(y)}$ describes a bidirectional causal relationship.