Shoppers push loaded up carts at a Target store in Culver City, California.

Consumer optimism slipped in December, after hitting historic highs in the previous month.

The Conference Board's measure of consumer confidence declined to 122.1 in December, further than the 128.1 anticipated by economists polled by Reuters.

In November, the index rose to 129.5, the highest mark since the index hit 132.6 in November 2000.

"The decline in confidence was fueled by a somewhat less optimistic outlook for business and job prospects in the coming months," Lynn Franco, Director of Economic Indicators at The Conference Board, said in a statement.

"Despite the decline in confidence, consumers' expectations remain at historically strong levels, suggesting economic growth will continue well into 2018," Franco said.

The index takes into account Americans' views of current economic conditions and their expectations for the next six months. Economists pay close attention to the numbers because consumer spending accounts for about 70 percent of U.S. economic activity.