Led by Åsa Samuelsson, MSc, of Karolinska Institutet, Stockholm, the researchers used a database including nearly 53,000 Swedish twins born between 1952 and 1958. Twin studies provide uniquely valuable information on the familial factors -- genetics and early life experiences -- affecting health and illness.

At follow-up from 1992 to 2007, the average percentage of participants on disability pension was 10.7 percent per year. Not surprisingly, the strongest risk factor for disability was age: risk was nine times higher for older versus younger workers. Disability pension rates were about five times higher for less-educated workers and two times higher for those who were unmarried.

Marital status was a stronger risk factor in men than women. Risk was also somewhat higher for people living in rural or semi-rural areas.

All of these factors remained significant after adjusting for familial factors. This indicates that disability risk is related to "factors not shared by family members, such as experience or choices in adulthood," the researchers write.

Recent decades have seen rising rates of sick leave and disability pensions in developed countries. Although some studies have looked at risk factors for disability pensions, the role of familial factors remains unclear. Studies of twins -- who share both genetic and (usually) early environmental factors -- provide a unique opportunity to tease out the factors contributing to risk.

The new study suggests that genetic and other familial factors play little if any role in long-term disability risk, Samuelsson and colleagues conclude. Rather, they believe the risk factors identified represent factors in later life that aren't necessarily shared by family members.

Note: If no author is given, the source is cited instead.

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