 Solution

Let $Z=1$ denote gestation under unrestricted sugar and $Z=0$ gestation under rationing, and let $Y_i(z)$ be person $i$'s potential outcome for later Type 2 diabetes under exposure $z$. A sharp causal null hypothesis isagainst the alternative that the two potential outcomes differ for at least one person. An average-effect formulation instead tests $H_0:\mathbb{E}[Y(1)-Y(0)]=0$ against a nonzero, or scientifically directed positive, average effect.