We're at the mercy of Google." Undecided voters in the US who turn to Google may see dramatically different views of the world – even when they're asking the same question.

Type in "Is Kamala Harris a good Democratic candidate", and Google paints a rosy picture. Search results are constantly changing, but last week, the first link was a Pew Research Center poll showing that "Harris energizes Democrats". Next is an Associated Press article titled "Majority of Democrats think Kamala Harris would make a good president", and the following links were similar. But if you've been hearing negative things about Harris, you might ask if she's a "bad" Democratic candidate instead. Fundamentally, that's an identical question, but Google's results are far more pessimistic.

"It's been easy to forget how bad Kamala Harris is," said an article from Reason Magazine in the top spot. Then the US News & World Report offered a positive spin about how Harris isn't "the worst thing that could happen to America", but the following results are all critical. A piece from Al Jazeera explained "Why I am not voting for Kamala Harris", followed by an endless Reddit thread on why she's no good.

You can see the same dichotomy with questions about Donald Trump, conspiracy theories, contentious political debates, and medical information. Some experts say Google is just parroting your own beliefs right back to you. It may be worsening your own biases and deepening societal divides along the way.

"We're at the mercy of Google when it comes to what information we're able to find," says Varol Kayhan, an associate professor of information systems at the University of South Florida in the US.

The bias machine

"Google's whole mission is to give people the information that they want, but sometimes the information that people think they want isn't the most useful," says Sarah Presch, digital marketing director at Dragon Metrics, a platform that helps companies tune their websites for better recognition from Google using methods known as "search engine optimization" or SEO.

It's a job that calls for meticulous combing through Google results, and a few years ago, Presch noticed a problem. "I started looking at how Google handles topics where there's heated debate," she says. "In a lot of cases, the results were shocking."

Some of the starkest examples looked at how Google treats certain health questions. Google often pulls information from the web and shows it at the top of results to provide a quick answer, which it calls a Featured Snippet. Presch searched for "link between coffee and hypertension". The Featured Snippet quoted an article from the Mayo Clinic, highlighting the words "Caffeine may cause a short, but dramatic increase in your blood pressure." But when she looked up "no link between coffee and hypertension", the Featured Snippet cited a contradictory line from the very same Mayo Clinic article: "Caffeine doesn't have a long-term effect on blood pressure and is not linked with a higher risk of high blood pressure".