Mortgage rates are expected to remain near historical lows through 2012, but the impact on home purchasing activity remains unknown until Americans see how the larger economy fares. Doug Duncan, chief economist with Fannie Mae, said positive indicators in the past few months,  including employment growth at the end of the year, would have to continue to improve before home purchasers feel comfortable jumping into the market for a new home. "If you start to see a pick up in growth then you could see improvement in mortgage activity," Duncan noted. Greg McBride, a senior financial analyst with Bankrate, projects mortgage interest rates will remain well under 5% and closer to the more recent 4%. He concedes it is possible rates could dip even further if the U.S. economy takes another hit. Freddie noted that the 30-year, fixed-rate mortgage hit 3.95% last week, while the 15-year, FRM averaged 3.24%. Duncan notes similar concerns about headwinds from the overall economy. "If people see the economy start to strengthen appreciably, they may want to take advantage of that and refinance" their mortgages, he said. "If there is a slow down in economic activity, that will hold rates lower longer and people who have been continually improving their credit record will be able to get into the refinance market on better terms." Duncan  and McBride both say purchase activity depends on economic growth. Write to Kerri Panchuk.