Millionaires became slightly more optimistic about the future of the U.S. economy in May, according to Spectrem's Millionaire Investor Index.

The index, which measures the investment outlook of households with $1 million or more in investable assets, rose five points to -9 last month, pulling it out of mildly bearish territory and into more neutral ground. The index hit a record low of -14 in April and June of this year; it is now at its third lowest level ever.

Millionaires cited rising gas and oil prices (40%) as a source of major concern about the U.S. economy. Nearly 20% said housing and real estate were their top concern, while 11% cited the results of the presidential election.

Affluent investors—those with $500,000 in investable assets—were slightly more concerned with rising gas prices (45%), while also noting housing and real estate (13%), election results (6%) and the economy (5%) as top concerns.

The Spectrem Affluent Investor Index, which measures the investment outlook of such investors, remained unmoved at -13, matching the index's second all-time low it reached in both January and April of this year. It continues to sit in mildly bearish territory, following its all-time low of -20 in March.

"With concerns over increasing gas and oil prices spiking for both groups in May, the investment sentiment of the wealthiest Americans is moving into the summer on anything but a high note," says George Walper Jr., president of Spectrem Group.

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