One of the best predictors of a man's income at age 50 is his height - at the age of 1.

Research published in today's edition of the Archives of Disease in Childhood found that baby boys who were undersized at 12 months, grew up to be smaller and less educated and had significantly lower incomes.

And, surprisingly, the association between height in infancy and income later in life does not appear to be linked to socio-economic status.

"Irrespective of social class at birth, men who grow slowly between birth and one year had lower income in later life," said David Barker of the environmental epidemiology unit at Southampton General Hospital in England.

The study does not explain why this is the case, but Dr. Barker said the most likely explanation is that slow physical growth might be accompanied by slow brain growth. Another possibility is that infants may be smaller because they are undernourished or often ill, factors that may lead to less physical activity and sensory stimulation.

Researchers also said they could not rule out the possibility that short stature as an adult led to discrimination and fewer opportunities for good-paying jobs.

More than 4,500 men were studied, all of whom were born in Helsinki between 1934 and 1944. Health records in Finland are kept impeccably and can be linked to other government databases, allowing this kind of research.

The researchers found that infant boys who measured 72 centimetres (28 inches) or less at one year grew up to have the least education and lowest incomes, while those measuring 80 centimetres (31.5 inches) or more were far more likely to pursue higher education and find well-paying jobs. Between those points, the correlation between infant size and adult income held fast.

The research is not the first to demonstrate the link between childhood size and educational success. A study of children born in Newcastle-on-Tyne in 1947 found that a strong predictor of IQ at 11 years of age was a child's height at 3. The taller children performed better in tests of reasoning, English and arithmetic, and went on to do better in school and to have higher incomes.

A number of studies have also shown tall men to have all manner of advantages. They marry sooner, are promoted more quickly, have higher incomes, and find more success in politics. For example, only three U.S. presidents have been below-average height. And a U.S. study showed that, during a 30-year career, a 5-foot-5 worker earns $160,000 less than his six-foot counterpart, which translates into about $800 an inch more annually for the taller worker.