How should a Pearson correlation coefficient of −1 be interpreted?

A Pearson correlation coefficient of −1 indicates a perfect linear relationship between two variables, often referred to as a perfect anticorrelation.12

In this context, the negative sign signifies the direction of the association: as one variable increases, the other decreases. Because the value is exactly −1, it demonstrates that the data points fall perfectly along a straight line with a negative slope, representing the strongest possible inverse linear relationship.12

It is important to remember that while this coefficient measures the strength and direction of a linear association, it does not establish a cause-and-effect relationship between the variables.1