The collective life evaluation score for the British fell to 43.6 in April, the lowest point so far this year, and down from a high of 52.3 recorded one year ago, Gallup officials said.
During the same time period, life ratings in the United States improved to 50.4 in April, up from 48.8 in 2011.
The Life Evaluation Index is a key component of the Gallup-Healthways Well-Being Index, which provides a comprehensive monthly measure of Americans' and Britons' physical, emotional and financial health.
The overall Life Evaluation Index score is calculated by subtracting the percentage of those suffering from the percentage of those thriving.
People are considered thriving if they rate their current lives a 7 or higher on a scale of 10, and expectations for their lives in five years an 8 or higher. People who rate their current or future lives a 4 or lower are classified as suffering.
All others are considered struggling.
Britons ages 18-29 had previously boasted the best life ratings, but in this survey, their ratings were essentially the same as those ages 30-44, while older Britons' life ratings were lower than those in younger age groups.
The Gallup-Healthways Well-Being Index survey of 16,769 British adults, conducted from Jan. 1-April 30, has a margin of error of 1 percentage point.
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