(HealthDay) -- It's graduation season, and new research offers yet another reason to congratulate someone who has completed at least nine years of education: They're likely to live longer.
An unusual, comprehensive study involving 1.2 million Swedes shows that students who were exposed to nine years of education rather than eight had a lower mortality rate after age 40. While the finding suggests an association between level of education and longevity, it does not establish a cause-and-effect link.
Those exposed to the additional year of education also had lower mortality from causes known to be related to education. The research, by Anton Lager of the Centre of Health Equity Studies and Jenny Torssander of the Swedish Institute for Social Research, both part of Stockholm University, was published in this week's issue of the Proceedings of the National Academy of Sciences.
Many studies have shown a link between more years of education and better health. But it has been difficult for researchers to show that the cause wasn't just personal characteristics, such as the ability to stick with school or delay gratification. It has also been hard to determine whether secondary benefits -- getting a better job or earning a higher income, for instance -- might be the reason why people with more education develop fewer health problems.
A policy change in Sweden provided a ready-made method to help solve those mysteries and answer the question: Are the number of years in school uniquely associated with better health?
From 1949 to 1962, Sweden added one year of compulsory schooling. Children continued to start school at age 7; the new program increased the number of years in school from eight to nine.
The change was implemented in a way that was designed to facilitate long-term evaluation of the value and impact of the additional year in school. All children born between 1943 and 1955 in 900 municipalities were included in the study. Each year, as the program was phased in, children in some municipalities were included in the new nine-year system and others were not. Those not included were the control group. The researchers gathered data on age of death and causes of mortality until 2007. During the 58-year follow-up period, about 92,000 of those in the study died due to various causes.
The authors found that in what they call "later adulthood," after age 40, the group with the ninth year of education had lower mortality from all diseases than did those with eight years in school. Those people also had a lower death rate from lung cancer, all cancers and accidents. Women with the ninth year of school were less likely to die from ischemic heart disease; men were less likely to die from external causes. All deaths of those in the study, except for the 2 percent to 3 percent who emigrated, were recorded.
Lager said the study shows the reason for lower mortality is not knowledge in and of itself. He speculated that the ninth year helped students develop a different attitude about themselves. "If your life is a little better, you take a little better care of yourself. If you make a little more income, have a job with a little more flexibility, more control of time, then maybe you use less tobacco and alcohol," he said.
Dr. Mark Cullen, a professor of medicine at Stanford University with research interests in social and environmental determinants of health, said "the study adds strong evidence that including additional years in school and higher education has a substantial impact on longevity."
Cullen believes the additional year contributed to the students' long-term ability to understand health messages, think effectively and manage their lives. "We should never discount the direct value of education in helping you interpret information and be involved in your own health," he said.
Explore further: Do well at school to avoid heart disease later, research shows
More information: The causal effect of education on mortality: 58-year follow-up of a quasi-experiment on 1.2 million Swedes, by Anton Lager and Jenny Torssander, PNAS, 2012.
For more on healthy aging, head to the U.S. National Institutes of Health.