Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 221 / 1 / b / i / Solution

The $X_1$ coefficient is the total causal effect of $X_1$ on $Y$. The node $X_1$ has no parents, so there is no backdoor path into it, and the unadjusted regression sums all directed paths from $X_1$ to $Y$.