Global banks appear to be giving top clients private information to win corporate-bond trading business, according to a new study showing the $56 billion-a-day market is stacked in favor of the most active investors with the broadest dealer networks.

The study examined the profitability of trades by insurance companies, which are whales in the credit world. It found that those with the best access to Wall Street’s bond-trading desks often had better outcomes than other investors ahead of market-moving events, such as mergers and acquisitions (M&A) and ratings downgrades.

Critically, the more important an insurer is to a dealer’s business, the more prescient their trades ahead of M&A transactions tend to be—especially if the dealer is part of a bank also involved in the takeover. The pattern holds even when taking into account the pricing benefits offered by large dealer networks, professors at Rice University, Yale University, and the University of Pennsylvania discovered.