Read: What Amazon thinks you’re worth

Amazon admits it takes profitability into account when displaying products, but it denies reworking its search tool to privilege its own products over the competition. In an emailed statement to The Atlantic, an Amazon spokesperson wrote, “The fact is that we have not changed the criteria we use to rank search results to include profitability. We feature the products customers will want, regardless of whether they are our own brands or products offered by our selling partners. As any store would do, we consider the profitability of the products we list and feature on the site, but it is just one metric and not in any way a key driver of what we show customers.”

Amazon’s carefully worded denial shows how difficult it is to prove that a search algorithm exhibits favoritism or discrimination. Amazon need not include a profitability metric in its formula to increase the profitability of its business. The Journal’s report alleges that Amazon’s search algorithms instead take into account proxies for profitability. The company makes more money from certain products than others, and if the more lucrative products have something in common (maybe where they’re sold or how they’re packaged), tracking that variable could theoretically scale with profitability. Engineers told the Journal that Amazon’s review committee wouldn’t approve any changes to the algorithm unless they also increased profitability.

Amazon is incredibly good at making money, which is why the possibility that it’s privileging its own products in its search tool is ringing alarm bells. Amazon controls the marketing, manufacturing, and distribution of its private-label products as well as the marketplace they’re sold on. Rigging the search algorithm, if the allegations are true, would only compound its advantages.

Online marketplaces have lately been under renewed scrutiny from federal regulators, both in the U.S. and abroad, as part of a larger conversation about breaking up tech companies and imposing greater fees for anticompetitive behavior. In 2017, the European Union fined Google’s parent company, Alphabet, $2.7 billion for allegedly pushing independent comparison-shopping sites to the second or third page of search results while displaying its own widget at the very top of results. Merchants pay Google when users click on these ads. And earlier this month, The New York Times reported that Apple reworked its App Store algorithm to unfairly privilege its own apps, suggesting Apple Music over Spotify when users searched for “music,” for example. (Apple denied this to the Times and, in an emailed statement to The Atlantic, reiterated that its algorithm takes a host of factors into account.)

Algorithms interpret potentially millions of data points, and the exact path from input to conclusion can be difficult to make plain. But the effects are clear. This is a very powerful asymmetry: Anyone can notice a change in search results, but it’s extremely difficult to prove what caused it. That gives algorithm designers immense deniability.