If the mathematical relationship between X and Y is very similar to the mathematical relationship between Z and Y, then what can we say about the causal relationship between X and Z? Please tell me. I have a background in social science and my brain seems to be unable to solve this puzzle.

Correlation does not imply causation, but it's suggestive in this particular case. Seeing as X and Y are objective variables (reproductive potential and age), and Z is something happening within minds (degree of grief), it's natural to hypothesize that Z arises from the mind observing X and Y or some correlates of them. Moreover, we can't help observing Y, and observing (correlates of) X has obvious evolutionary benefit so we're probably doing that anyway. In light of this, what alternative causal hypotheses do you have for explaining the relationship between X, Y and Z?