As Massachusetts' housing crisis lingers, a new study has found that investors bought up a significant portion of the state's housing stock, specifically in minority neighborhoods, effectively pushing out other buyers.

Between 2004 and 2018, 21% of transactions in Greater Boston came from investors. Investor activity in the region rose rapidly from 16% in 2004 to 23% in 2018, according to a report published by the Metropolitan Area Planning Council.

More than half of those purchases were paid for in cash, making them highly attractive bids for sellers.

"Cash buying puts investors at an advantage over non-investors because cash sales are typically more attractive to sellers, who then do not have to deal with financing paperwork and can therefore close on their sale faster," the report says.

The investments weren't distributed equally throughout the metro, the report found. Investors targeted "high-density urban submarket with relatively low housing prices." Sales in those areas accounted for 31% of investor purchases.

These types of investments ramped up after the 2008 Global Financial Crisis, with two- and three-family properties seeing the most interest from investors because of the high rates of foreclosure associated with them during that period.

"These trends together suggest that not only were investors collecting foreclosed properties through the recession period, but that those with money also saw the recession and years of recovery following it as an opportunity to get into the Greater Boston housing market at a time with relatively lower prices and potential for significant portfolio and profit growth," the report says.

Once investors purchased properties, they were also more likely to flip them, with large and institutional investors most likely to resell at a profit. Nearly half of single-family homes and a fifth of two-family homes were flipped by investors, while 8% and 9%, respectively, were flipped by noninvestors.

The report highlighted a number of policy pitches that could help slow large investor purchases and help renters, including rent caps and expansion of housing aid options.

In the past few years, local and state politicians have pushed more aggressively to bring housing to Massachusetts. Those efforts include former Gov. Charlie Baker's MBTA Communities law, Gov. Maura Healey's $4B housing bill, and rent stabilization and affordable housing investments made by Mayor Michelle Wu's administration.