People who drift into teaching because they have nothing else to do are just as committed to the profession as colleagues who view teaching as their vocation.
That was the finding of a government-funded study that divided teachers into those who join the profession "by design" and those who fall into it "by default".
Researchers from the Institute for Public Policy Research interviewed 73 secondary teachers to find out what attracted them, or dissuaded them, from teaching in challenging schools.
They defined the default teachers as including "older female teachers who remember being advised that teaching was a 'good job for women' and who opted for teaching over a career in banking and the civil service".
"This category also includes younger teachers who enter the profession after university and who do not know what else to do."
The "by design" teachers were those who had their hearts set on becoming teachers before they went to university or who had switched careers because they realised they would find teaching more satisfying.
The researchers found some default teachers were "less likely to articulate a positive account of teaching as a profession of value and which has real impact". But the default teachers were no less committed to their jobs.
The report noted that by-design and by-default teachers appeared to be evenly spread among the schools studied, half of which were considered struggling because fewer than a quarter of their pupils gained five A* to C grade GCSEs.
The teachers' motivations had often changed over time, with some who fell into teaching developing a passion for the job, while some originally keen teachers found that other factors in their lives became greater priorities.
Miranda Lewis, a member of the IPPR research team, said: "Many of the most burnt-out teachers we spoke to were actually the ones who had the highest expectations, rather than the ones who went into teaching more casually."
Famous figures who admit they drifted into teaching include the bestselling authors Philip Pullman and Roddy Doyle, television presenter Chris Tarrant, and fashion designer Vivienne Westwood.
The report suggests that exchange schemes should be introduced so staff from academically successful and struggling schools can swap jobs for a few weeks.
It also recommends that challenging schools should focus their recruitment campaigns on how inspiring the job is.
The Teacher Training Agency tried this approach with its most recent advertising campaign, which included such slogans as "work with the most exciting people in the country". But applications to teacher training courses still fell by 1 per cent last year.
The Why here? report is at www.dfes.gov.uk.