A second set of considerations requires policymakers to examine two further questions. The first is whether Amazon’s market share in specific product categories is sufficient to give it market power that might not be in consumers’ long-term interests, and is not reflected in its low share of the total retail market.

Here the evidence is unclear. Yes, Amazon has cut the retail price of books and music. But lower prices now are not necessarily in the interests of consumers if they harm competition in the long run. What we do not know, and an antitrust investigation might discover, is whether these prices are predatory—to be raised once competition is more or less eliminated. (We could also test whether there is a relationship between Amazon’s margins on products and its share of the markets in those products.)

The second area of possible inquiry involves Amazon’s pricing and other business policies to determine whether it uses financial muscle, rather than mere efficiency, to nip would-be competitors in the bud.