Twitter is investigating after users discovered its picture-cropping algorithm sometimes prefers white faces to black ones.

Users noticed when two photos – one of a black face the other of a white one – were in the same post, Twitter often showed only the white face on mobile.

Twitter said it had tested for racial and gender bias during the algorithm’s development.

But it added: “It’s clear that we’ve got more analysis to do.”

Twitter’s chief technology officer, Parag Agrawal, tweeted: “We did analysis on our model when we shipped it – but [it] needs continuous improvement.

“Love this public, open, and rigorous test – and eager to learn from this.”

The latest controversy began when university manager Colin Madland, from Vancouver, was troubleshooting a colleague’s head vanishing when using videoconference app Zoom.

The software was apparently mistakenly identifying the black man’s head as part of the background and removing it.

His discovery prompted a range of other experiments by users, which, for example, suggested:

Responding to criticism, he tweeted: “I know you think it’s fun to dunk on me – but I’m as irritated about this as everyone else. However, I’m in a position to fix it and I will.

“It’s 100% our fault. No-one should say otherwise.”

Zehan Wang, a research engineering lead and co-founder of the neural networks company Magic Pony, which has been acquired by Twitter, said tests on the algorithm in 2017, using pairs of faces belonging to different ethnicities, had found “no significant bias between ethnicities (or genders)” – but Twitter would now review that study.

“There are many questions that will need time to dig into,” he said.

“More details will be shared after internal teams have had a chance to look at it.”

Twitter has automatically activated extra account protection for politicians and key figures in the forthcoming US election.

A select group of election-related accounts will have to make immediate changes to improve their security.

Twitter was the victim of one of the most significant hacks of a social network in history, in June.

It said it had learned “from the experience of past security incidents” and was focused on election security.

Starting this week, those accounts will start receiving in-app notifications of immediate changes on the user’s side.

Under the new rules, they will need to use a strong password – those that do not meet Twitter’s standards will be required to change it the next time they log in.

All the accounts have had password reset protection enabled by default – a safety measure that requires the account holder to confirm the email address and password on file if they want to reset their password.

And they will also be encouraged – but not required – to enable two-factor authentication.

The June hack saw some of the world’s most notable celebrities – such as Bill Gates, Joe Biden and Kanye West – have their accounts seized by the attacker to tweet an apparently simple Bitcoin scam.

But one of Twitter’s most-followed accounts – US President Donald Trump – was unaffected. It later emerged that this was because the president’s account had extra security protections in place internally.

The change comes at a time when Twitter is locked in a battle against electoral misinformation on the platform – one that has put it at odds with President Trump.

It had previously tagged some of the tweets for breaching policies on glorifying violence and abusive behaviour.

But the wider concern about the role of the social media companies in the election stems from the 2016 presidential campaign, which saw President Trump win his first term in office.

Nintendo has discontinued its 3DS handheld after about 76 million sales over a nine-and-a-half year period.

A notice on the Japanese firm’s site says “manufacturing of the Nintendo 3DS family of systems has ended”.

The device had the ability to trick the human eye into seeing 3D images like those in some cinema screenings – but without special glasses.

However, its launch received a lukewarm reception and it only gained popularity later.

The console’s demise has long been expected. Last year, the company said it no longer planned to make any new first-party games for the system.

It means the original Nintendo DS retains the title of being the bestselling mobile console.

And the Nintendo Switch – a hybrid handheld-and-home machine – is the current focus of Nintendo’s efforts.

The unique selling point of the 3DS was its stereoscopic 3D effect coming from the screen itself, turned on with a simple slider.

The optical illusion only works when the screen is in a very specific spot in front of the face – but exactly where gamers tend to hold handheld consoles.

It was impossible to showcase on video or photo but impressed many who tried it.

The console was packed with experimental features – augmented reality games, dual 3D cameras, and the the ability to detect nearby consoles while in low-power standby, called “Streetpass” – which allowed players to connect with strangers without exchanging any information.

“The 3DS and its StreetPass functionality were driving factors for the founding of many of the groups now found in [Nintendo Players UK],” said player John Edwards. “Without the 3DS I doubt we’d have such a thriving community.”

Despite all the bells and whistles, the 3DS did not start well. A high asking price for the time of over £200, coupled with a lacklustre list of initial games, hurt sales. The fact that the key feature of 3D had to be tried in person was another obstacle.

Within six months of the launch, Nintendo announced a major price cut to between £100 and £150. Coupled with more games – including a beloved remake of the classic Ocarina of Time – the fortunes of the handheld started to change.

“It became a fantastic Nintendo machine for legacy content,” recalls games writer Nathan Ellingsworth.

“It was how I experienced Majora’s Mask, Earthbound, Minish Cap and many more for the first time and it had its own steady stream of incredible titles.”

The 3DS quickly came into its own, with dedicated Mario titles and new Pokemon games in addition to its growing back catalogue.

It was revamped and relaunched several times – on top of the “XL” models, the “New 3DS” had increased horsepower, while the 2DS dropped the standout 3D feature entirely for those who saw it as a gimmick.

By June 2020, the entire system family had sold nearly 76 million units, Nintendo says – far outclassing its home console of the same era, the WiiU, which sold less than 14 million.

But the modern Nintendo Switch, released in 2017, has already sold more than 60 million units, and can be played as a handheld.

While Nintendo initially insisted it was not a “replacement” for the 3DS, declining support and sales meant the venerable handheld’s time was limited. In the year to March 2020, only 69,000 3DS consoles were sold.