As many of his friends struggled to find jobs in 2009, Rand Ghayad, a PhD candidate in economics at Northeastern University, decided to search for answers to explain why smart, educated, and experienced people remained out of work for so long, despite hundreds of thousands of job vacancies.

What he found after more than four years of research has challenged conventional assumptions about long-term unemployment and shifted the national debate on what many analysts and policy makers see as one of the most pressing problems facing the US economy.

The Federal Reserve Bank of Boston has published his work, and he has been cited by several influential economists, including Paul Krugman, the New York Times columnist and Nobel laureate. And at a recent conference of top labor economists in Washington, Ghayad’s findings were the frequent topic of discussion.

“You research something for a very long time and you don’t know if it’s going to go somewhere or not,” said Ghayad, 27, who expects to complete his degree at the end of the summer. “I was very lucky.”

Four years after the last recession ended, nearly 4 million Americans — roughly one-third of all unemployed — have been out of work for more than year. Traditionally, economists have blamed long-term unemployment on long-lasting jobless benefits that undermine incentives to find work, citing European nations where both extensive social safety nets and long-term unemployment are entrenched.