UNITED KINGDOM — More than a third of Britons say they have changed social class, according to a study by research firm Attest. The study coined the term "polyclass" to describe the equivalent of 6 million British people who identified as belonging to more than one social class at once.

Upper-middle and upper-class people were the most likely to identify as belonging to more than one social class, the study found. Seventy percent of working-class people said they were in the same social category they were born into.

Nearly half of respondents said they had felt judged for their social class, and most ranked social class above age, gender, ethnicity and sexual orientation in terms of how they think others see them. Almost half of millennials said they had become a different social class or identified as belonging to more than one social class. Working-class people were more likely to say they changed how they behaved or spoke to fit in with other social or professional groups, although a third said they had never felt the need to change to fit in.

Dominic Abrams, professor of social psychology and director of the Centre for the Study of Group Processes at the University of Kent, said traditional class frameworks were losing relevance. "The supposed class alignment with different political parties seems to be breaking down. So traditional categorizations are not terribly helpful any more," he said.

Abrams also described how class background can shape interactions in new environments. "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," he said.

Separate research by the Sutton Trust found a happiness gap between social classes, with working-class people who went on to get good jobs not catching up with their middle or upper-class peers in levels of happiness. People from working-class backgrounds were a third more likely to experience low wellbeing than those from professional backgrounds. Those who moved into higher status jobs became happier, but those from more affluent backgrounds still had the highest levels of wellbeing and were protected from the effects of moving downwards, probably because they had a financial safety net.

Nick Harrison, chief executive of the Sutton Trust, said: "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."

Harrison also said wellbeing reflected a wider range of factors. "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," he said.