Consider again our little world. As long as we have two people, freedom of exchange is efficient whether we use Marshall’s definition (net gains) or Pareto’s (some gain, no loss). At any price between fifty cents and a dollar both Mary and John gain. But now put Anne back into the picture—with a value for the apple of only seventy-five cents. Anne proposes, on the principle of gender solidarity, a new legal rule: Women can trade only with other women. Going from gender solidarity to freedom of exchange produces a net gain by Marshall’s criterion, since it means John instead of Anne getting the apple, and it is worth more to him than to her. But it makes Anne worse off, so it is not a Pareto improvement.