More than a third of Britons say they have changed social class, with upper-middle and upper-class people most likely to identify as belonging to more than one class, according to a survey.

Working-class people were the least likely to say they had changed class or identified with more than one, with 70% saying they were in the same social category they were born into, the study by research firm Attest found.

Researchers coined the term “polyclass” to describe the equivalent of 6 million British people who identified as belonging to more than one class at once.

The survey of 2,000 people also found an entrenched sensitivity about the topic, with nearly half of respondents saying they had felt judged for their class, and most ranking social class above age, gender, ethnicity and sexual orientation in terms of how they think others see them.

Dominic Abrams, a professor of social psychology and director of the Centre for the Study of Group Processes at the University of Kent, said he thought the term “polyclass” was useful at a time when class boundaries were far more blurred than in previous generations.

Traditional categorisations such as ABC1, denoting the middle classes, were no longer necessarily signals of belief or attitude, and not even necessarily signs of class.

“The supposed class alignment with different political parties seems to be breaking down. So [traditional categorisations are] not terribly helpful any more,” Abrams said. “I think the broader perception now is that if you want to move around, and you have the resources to do so, you can. A lot of people say they have moved class.”

This viewpoint is particularly apparent among millennials, according to the survey, with almost half saying they had become a different class or identified as belonging to more than one. Working-class people were the least likely to have moved class.

Abrams said: “Working-class people tend to have more enduring and stable cultural roots because they can be rooted more in place. It’s a value they’re more likely to reinforce. The question is whether they can use it as a tool in their armoury for negotiating the environments that they’re entering into or whether it becomes a burden because they feel judged because of it.”

Working-class people were more likely to say they changed how they behaved or spoke to fit in with other social or professional groups, but a significant third of working-class people said they had never felt the need to change to fit in with another group.

Separate research from the Sutton Trust has found there is a “happiness gap” between social classes and that working-class people who went on to get good jobs never caught up with their middle or upper-class peers in the happiness stakes.

People from working-class backgrounds were a third more likely to experience low wellbeing than those from professional backgrounds, the report said.

Those who moved into higher status jobs did become happier, but the research found that those who came from more affluent backgrounds still enjoyed the highest levels of wellbeing. They were also protected from the effects of moving downwards, probably because they had a financial safety net, researchers said.

Nick Harrison, the chief executive of the Sutton Trust, said social mobility “hugely improves people’s lives, but it doesn’t always guarantee happiness”.

He added: “Even when people climb the social ladder, their long-term wellbeing is still shaped by where they started. Opportunity and life satisfaction are about far more than just pay, promotions and property. Family and friends, education levels, community ties and work-life balance all play their part.

“Where you come from shouldn’t determine how happy your life turns out. To break this link, we need to tackle inequality in education, open up access to careers and invest in communities. Opportunities to get on in life and enjoy greater levels of wellbeing should be genuinely open to everyone.”