Richard's developmental psychology class finds a positive correlation between the height and weight of the class members. A class member asks, "What does early nutrition have to do with this finding?" This class member has stumbled onto what issue associated with correlations?

The third-variable problem

Explanation:

The third variable problem refers to the fact that a causal relationship between two variables cannot be inferred from the naturally occurring correlation between them because of the ever present possibility of third variable correlation.

Also, the third variable is the entire cause of the correlation between the two variables of interest and there's no causal relationship between them at all but the key difference between experiments and correlational studies concerns the variable that is thought to be the cause.