There are about 3.4 million U.S. parents sharing a home with their adult children in the United States, up from 2.1 million in 2000, and the rise is attributed to a host of factors including joblessness, foreclosed homes and the high cost of living, according to the study released on Tuesday by research company Twenty something Inc.

The numbers are expected to go up as living with parents has become more acceptable compared with several years ago when young adults looked forward to reaching 18 and living independently, it said.

The arrangement requires adjustment from both parents and returning adult children.

Boston College psychology professor Joseph Tecce said parents must acknowledge their offspring are no longer young children, but companions who help run the household.

"Parents are not running a hotel. Everyone should say this is a chance for us to enter into a cooperative venture. We love each other and we're going to help each other out," he said.