The millionaires’ club in the United States grew by 16 percent in 2009, following a 27 percent decline in 2008. Families with a net worth of at least $1 million, excluding primary residences, rose to 7.8 million in 2009, an increase from 6.7 million a year earlier, according to a survey of high-net-worth U.S. households conducted by Spectrem Group. The Standard & Poor’s 500 Index increased 24 percent in 2009 and has risen 68 percent over the past 12 months.

Affluent households, which the survey defined as those with net assets of $500,000 or more, increased 12 percent to 12.7 million, the Chicago-based consulting firm said in a statement Tuesday. The number of households with a net worth of more than $5 million rose 17 percent to 980,000.

The average age of a so-called affluent investor is 58, compared with 62 for a millionaire and 67 for an investor with more than $5 million. Survey respondents in all three categories said they were most concerned about the impact of a prolonged economic decline on their financial well-being, according to Spectrem. The highest number of affluent and millionaire investors said they were likely to invest in cash, which includes certificates of deposit, over the next 12 months, followed by stocks and then bonds, said Spectrem.

The biggest number of ultra-high-net-worth investors said they were likely to invest in equities, followed by cash and international investments. The fewest wealthy investors said they were likely to invest in alternative investments such as hedge funds and investment real estate, Spectrem said.

The increase in the number of millionaires is still being held back by residential and investment real estate, which hasn’t bounced back, Walper said. The S&P/Case-Shiller index of home prices in 20 major cities was down 29 percent in December from its July 2006 peak.

This may further explain the renewed strength of the Manhattan luxury real estate market.