Most Americans believe that there is a problem with obesity, a problem that is spreading across the globe. But, oddly, despite the fact that obesity numbers have continued upward for years, Americans are in denial. Americans often think that obesity statistics are improving, and that less people are obese, when in fact the opposite is true. Americans also think that they have lost weight since last year, when in fact they gained some weight.

The Behavioral Risk Factor Surveillance System is a survey conducted by telephone that contacts a cross-section of 400,000 adults, with assistance from the Centers for Disease Control and Prevention. The system collects data on a monthly basis in all 50 states, the District of Columbia, the U.S. Virgin Islands, Puerto Rico, and Guam.

Lead authors Catherine Wetmore and Ali Mokdad used the information from the survey from 2008 and 2009 to identify how Americans perceived their weight changes. The researchers calculated participants' body mass index (BMI) from their reported height and weight. Participants also told pollsters about their believed weight and height the year prior.

If the reported weight changes were to be believed, the obesity prevalence in the United States should have decreased by 2.0 percent for men and 0.9 percent for women. In reality, the obesity prevalence in the United States increased by 0.4 percent, meaning 4.4 million obese Americans were missing from perceived weight loss to what was actually reported.

Women over 50 were particularly off in their weight loss perceptions, believing that they had lost an average of two pounds more than they had. People with chronic diseases, who are tasked by doctors to lose weight, were also erroneous. They believed that they had lost an average of four or more pounds.

Researchers are uncertain about the cause of the discrepancy. Wetmore suspects that the cause may be rooted in vanity, but says that they had not conducted research on that aspect.

The study does have its limitations, of course. The Behavioral Risk Factor Surveillance System calls random households, meaning that the majority of people are not surveyed twice. Such a survey would be made better if there was a way to assess the actual weight of people from year to year, and measure that against their reported weight gains and losses. In addition, partly since many people do not answer phone calls from unknown numbers, only 9 percent of people answer surveys. Despite the CDC's best intentions, no survey will be as accurate a picture of the population as we would like.

Obesity is associated with a number of chronic illnesses, like diabetes and cardiac issues. The World Health Organization estimates that 1.4 billion adults are overweight and an additional 500 million people are obese.

The study was published in Preventative Medicine journal.