The lack of a secondary education is a powerful predictor of early death, according to a new US study.
The death rate per head of population increases in communities with high numbers of people who did not complete secondary school.
The new findings are published this morning in the British Medical Journal.
The study looks past the usual assumptions about higher death rates caused by poverty and finds that a lack of formal education is a better indicator of early mortality.
Prof Andreas Muller of the University of Arkansas analysed US census data from 1989 and 1990 from all 50 states and the District of Columbia.
He calculated mortality from all causes, and also examined variables such as income and the number of people who did not complete high school.
Earlier studies had highlighted the link between income inequality and mortality. These showed that poverty leads to poorer health and higher death rates over time.
Prof Muller decided to examine whether income inequality was simply a manifestation of other socio-economic conditions that could influence how long a person lived.
"Among those variables, the contribution of formal education deserves most attention since it typically precedes and predicts work and income," he wrote in the BMJ.
Surprisingly, he found that income inequality had no effect on age at death when the level of formal education was taken into account.
"Educational attainment was a more powerful predictor of differences in mortality than income inequality in US states," he said.
In his study, the percentage of people who did not complete secondary school ranged from about 15 per cent to 35 per cent.
Prof Muller found that a 20 per cent rise in this number pushed up the number of deaths per 1,000 population by 2.1.
His study suggested that "lack of high-school education was related to lack of health insurance, belonging to economically de- pressed minority groups, working in jobs with high risk of injury and smoking.
"This finding suggests that lack of material resources, occupational exposure to risk and certain learnt risk behaviour might be reflected in the large education-mortality effect."
While acknowledging the limitations of his study, Prof Muller believes it indicates that lack of education accounts for the income inequality effect.
