Is there evidence that recent consolidation in the health insurance industry has adversely affected premiums?

James Robinson suggests that recent consolidation in the insurance market has been a cause of higher health insurance prices (premiums). Although the recent consolidation among health insurers and rising premiums are indisputable, it is unlikely that consolidation has had any adverse effect on premiums nationwide, and Robinson provides no data that suggest otherwise. Specifically, he does not present data showing an increase in concentration in any relevant market during the past few years, let alone any resulting increase in premiums. Health insurance consolidation in certain local markets could adversely affect premiums, but it seems clear that it is not a major national antitrust issue.