Boston ranked 20th on a list of the 25 real estate markets at greatest risk of being in a bubble, according to a recently published article by the Urban Institute.

“We think of a housing bubble as house price growth that isn’t sustainable because it isn’t consistent with underlying fundamentals, like income and job growth,” the authors wrote.

The paper says the country as a whole is not in a real estate bubble, but some local markets might be as home prices become less and less affordable to residents with the area median income, suggesting those high prices are based more on speculation and are therefore less sustainable.