Researchers led by Archana Singh-Manoux from the Centre for Research in Epidemiology and Population Health in France and University College London observed 5,198 men and 2,192 women over a 10-year period from 1997.[break]
The volunteers were London civil servants aged between 45 and 70 who had been enrolled in a long-term health study.
Over the 10 years, the participants were tested three times -- for memory, vocabulary, and skills in aural and visual comprehension.
During this time, there was a 3.6-percent decline in mental reasoning in men aged 45-49 and a 9.6-percent fall in those aged 65-70. The corresponding figures for women were 3.6 and 7.4 percent.
"Cognitive decline is already evident in middle age," says the paper, which defines this as the years from 45 to 49.
The findings should spur further research into spotting and braking cognitive deterioration, the authors hope.
Many societies face an "exponential increase" in the number of elderly people as a result of increases in life expectancy, they note.
"These changes are likely to have a profound influence on individuals´ lives and society at large. Poor cognitive status is perhaps the single most disabling condition in old age."
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