Google may be favouring certain US presidential hopefuls over others, according to an analysis of how Gmail sorts incoming emails.

The study, conducted by The Markup, suggested Democratic candidate Pete Buttigieg received a significant boost compared to other leading candidates, as his campaign emails were the most likely to be directed to Gmail’s Primary inbox folder.

Other candidates, including Bernie Sanders and Joe Biden, had nearly all of their emails directed to either the Promotions or Spam folder of Gmail.

The investigation was carried out between October and February using a fresh Gmail account signed up to the mailing lists of 16 different presidential candidates.

Over 5,000 emails were received in this period, with 90 per cent of the incoming mail automatically filed under either Promotions or Spam.

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The results appear to reveal a bias towards the former Indiana mayor, who was the only presidential candidate whose campaign emails were delivered to the Primary inbox more than 50 per cent of the time.

However, Google’s secretive algorithm means other factors could be behind Mr Buttigieg’s emails being sorted more favourably.

Google does not reveal how Gmail determines which folder an email is directed to and did not give specific details on how its algorithm may have influenced the results of the study.

A spokesperson said the categories “help users organise their email”, adding that “mail classifications automatically adjust to match users’ preferences and actions”.

A promotional video explaining how Gmail works claims the Primary inbox is reserved for emails users “really, really want”, while Promotions is usually earmarked for emails from companies.

A variety of factors could potentially determine which tab an email falls under, including a user’s location, browsing history and Google profile.

The Gmail account used in the study used Tampa, Florida, as its location, and was only used to sign up to candidates’ mailing lists.

Other factors, such as the device used to access the account, web browser or operating system were not listed in the study. It is therefore possible metadata could have influenced the algorithm’s decision to favour some candidates over others.